Protozoa

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Clockwise from top left: Blepharisma japonicum, a ciliate; Giardia muris, a parasitic flagellate; Centropyxis aculeata, a testate (shelled) amoeba; Peridinium willei, a dinoflagellate; Chaos carolinense, a naked amoebozoan; Desmerella moniliformis, a choanoflagellate

Protozoa (also protozoan, plural protozoans) is an informal term for single-celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microorganisms or organic tissues and debris.[1][2] Historically, the protozoa were regarded as "one-celled animals", because they often possess animal-like behaviours, such as motility and predation, and lack a cell wall, as found in plants and many algae.[3][4] Although the traditional practice of grouping protozoa with animals is no longer considered valid, the term continues to be used in a loose way to identify single-celled organisms that can move independently and feed by heterotrophy.

In some systems of biological classification, Protozoa is a high-level taxonomic group. When first introduced in 1818, Protozoa was erected as a taxonomic class,[5] but in later classification schemes it was elevated to a variety of higher ranks, including phylum, subkingdom and kingdom. In a series of classifications proposed by Thomas Cavalier-Smith and his collaborators since 1981, Protozoa has been ranked as a kingdom.[6][7][8] The seven-kingdom scheme presented by Ruggiero et al. in 2015, places eight phyla under Kingdom Protozoa: Euglenozoa, Amoebozoa, Metamonada, Choanozoa sensu Cavalier-Smith, Loukozoa, Percolozoa, Microsporidia and Sulcozoa.[9] Notably, this kingdom excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, dinoflagellates, foraminifera, and the parasitic apicomplexans, all of which are classified under Kingdom Chromista. Kingdom Protozoa, as defined in this scheme, does not form a natural group or clade, but a paraphyletic group or evolutionary grade, within which the members of Fungi, Animalia and Chromista are thought to have evolved.[9]

Contents

- 1History

- 2Characteristics

- 2.1Size

- 2.2Habitat

- 2.3Feeding

- 2.4Motility

- 2.5Pellicle

- 2.6Life cycle

- 3Classification

- 4Ecology

- 4.1Disease

- 5References

- 6Bibliography

- 7External links

History[edit]

Class Protozoa, order Infusoria, family Monades by Georg August Goldfuss, c. 1844

The word "protozoa" (singular protozoon or protozoan) was coined in 1818 by zoologist Georg August Goldfuss, as the Greek equivalent of the German Urthiere, meaning "primitive, or original animals" (ur- ‘proto-’ + Thier ‘animal’).[10] Goldfuss created Protozoa as a class containing what he believed to be the simplest animals.[5] Originally, the group included not only single-celled microorganisms but also some "lower" multicellular animals, such as rotifers, corals, sponges, jellyfish, bryozoa and polychaete worms.[11] The term Protozoa is formed from the Greek words πρῶτος (prôtos), meaning "first", and ζῶα (zôa), plural of ζῶον (zôon), meaning "animal".[12][13] The use of Protozoa as a formal taxon has been discouraged by some researchers, mainly because the term implies kinship with animals (Metazoa)[14][15] and promotes an arbitrary separation of "animal-like" from "plant-like" organisms.[16]

In 1848, as a result of advancements in cell theory pioneered by Theodor Schwann and Matthias Schleiden, the anatomist and zoologist C. T. von Siebold proposed that the bodies of protozoans such as ciliates and amoebae consisted of single cells, similar to those from which the multicellular tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only such unicellular forms, to the exclusion of all metazoa (animals).[17] At the same time, he raised the group to the level of a phylum containing two broad classes of microorganisms: Infusoria (mostly ciliates and flagellated algae), and Rhizopoda (amoeboid organisms). The definition of Protozoa as a phylum or sub-kingdom composed of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology"[18]—and the term came into wide use.

John Hogg's illustration of the Four Kingdoms of Nature, showing "Primigenal" as a greenish haze at the base of the Animals and Plants, 1860

As a phylum under Animalia, the Protozoa were firmly rooted in the old "two-kingdom" classification of life, according to which all living beings were classified as either animals or plants. As long as this scheme remained dominant, the protozoa were understood to be animals and studied in departments of Zoology, while photosynthetic microorganisms and microscopic fungi—the so-called Protophyta—were assigned to the Plants, and studied in departments of Botany.[19]

Criticism of this system began in the latter half of the 19th century, with the realization that many organisms met the criteria for inclusion among both plants and animals. For example, the algae Euglena and Dinobryon have chloroplasts for photosynthesis, but can also feed on organic matter and are motile. In 1860, John Hogg argued against the use of "protozoa", on the grounds that "naturalists are divided in opinion—and probably some will ever continue so—whether many of these organisms, or living beings, are animals or plants."[14] As an alternative, he proposed a new kingdom called Primigenum, consisting of both the protozoa and unicellular algae (protophyta), which he combined together under the name "Protoctista". In Hoggs's conception, the animal and plant kingdoms were likened to two great "pyramids" blending at their bases in the Kingdom Primigenum.

Six years later, Ernst Haeckel also proposed a third kingdom of life, which he named Protista. At first, Haeckel included a few multicellular organisms in this kingdom, but in later work he restricted the Protista to single-celled organisms, or simple colonies whose individual cells are not differentiated into different kinds of tissues.

Despite these proposals, Protozoa emerged as the preferred taxonomic placement for heterotrophic microorganisms such as amoebae and ciliates, and remained so for more than a century. In the course of the 20th century, however, the old "two kingdom" system began to weaken, with the growing awareness that fungi did not belong among the plants, and that most of the unicellular protozoa were no more closely related to the animals than they were to the plants. By mid-century, some biologists, such as Herbert Copeland, Robert H. Whittaker and Lynn Margulis, advocated the revival of Haeckel's Protista or Hogg's Protoctista as a kingdom-level eukaryotic group, alongside Plants, Animals and Fungi.[19] A variety of multi-kingdom systems were proposed, and Kingdoms Protista and Protoctista became well established in biology texts and curricula.[20][21][22]

While many taxonomists have abandoned Protozoa as a high-level group, Thomas Cavalier-Smith has retained it as a kingdom in the various classifications he has proposed. As of 2015, Cavalier-Smith's Protozoa excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, dinoflagellates and foraminifera (all members of the SAR supergroup). In its current form, his kingdom Protozoa is a paraphyletic group which includes a common ancestor and most of its descendants, but excludes two important clades that branch within it: the animals and fungi.[9]

Since the protozoa, as traditionally defined, can no longer be regarded as "primitive animals" the terms "protists", "Protista" or "Protoctista" are sometimes preferred. In 2005, members of the Society of Protozoologists voted to change its name to the International Society of Protistologists.[23]

Characteristics[edit]

Size[edit]

Protozoa, as traditionally defined, range in size from as little as 1 micrometre to several millimetres, or more.[24] Among the largest are the deep-sea–dwelling xenophyophores, single-celled foraminifera whose shells can reach 20 cm in diameter.[25]

The ciliate Spirostomum ambiguum can attain 3 mm in length

Species

Cell type

Size in micrometres

Plasmodium falciparum

malaria parasite, trophozoite phase[26]

1-2

Massisteria voersi

free-living cercozoan amoeboid[27]

2.3–3

Bodo saltans

free living kinetoplastid flagellate[28]

5-8

Plasmodium falciparum

malaria parasite, gametocyte phase[29]

7-14

Trypanosoma cruzi

parasitic kinetoplastid, Chagas disease[30]

14-24

Entamoeba histolytica

parasitic amoebozoan[31]

15–60

Balantidium coli

parasitic ciliate[32]

50-100

Paramecium caudatum

free-living ciliate[33]

120-330

Amoeba proteus

free-living amoebozoan[34]

220–760

Noctiluca scintillans

free-living dinoflagellate[35]

700–2000

Syringammina fragilissima

foraminiferan amoeboid[25]

up to 200000

Habitat[edit]

Free-living protozoans are common and often abundant in fresh, brackish and salt water, as well as other moist environments, such as soils and mosses. Some species thrive in extreme environments such as hot springs[36] and hypersaline lakes and lagoons.[37] All protozoa require a moist habitat; however, some can survive for long periods of time in dry environments, by forming resting cysts which enable them to remain dormant until conditions improve.

Parasitic and symbiotic protozoa live on or within other organisms, including vertebrates and invertebrates, as well as plants and other single-celled organisms. Some are harmless or beneficial to their host organisms; others may be significant causes of diseases, such as babesia, malaria and toxoplasmosis.

Isotricha intestinalis, a ciliate present in the rumen of sheep.

Association between protozoan symbionts and their host organisms can be mutually beneficial. Flagellated protozoans such as Trichonympha and Pyrsonympha inhabit the guts of termites, where they enable their insect host to digest wood by helping to break down complex sugars into smaller, more easily digested molecules.[38] A wide range of protozoans live commensally in the rumens of ruminant animals, such as cattle and sheep. These include flagellates, such as Trichomonas, and ciliated protozoa, such as Isotricha and Entodinium.[39] The ciliate subclass Astomatia is composed entirely of mouthless symbionts adapted for life in the guts of annelid worms.[40]

Feeding[edit]

All protozoans are heterotrophic, deriving nutrients from other organisms, either by ingesting them whole or consuming their organic remains and waste-products. Some protozoans take in food by phagocytosis, engulfing organic particles with pseudopodia (as amoebae do), or taking in food through a specialized mouth-like aperture called a cytostome. Others take in food by osmotrophy, absorbing dissolved nutrients through their cell membranes.[citation needed]

Parasitic protozoans use a wide variety of feeding strategies, and some may change methods of feeding in different phases of their life cycle. For instance, the malaria parasite Plasmodium feeds by pinocytosis during its immature trophozoite stage of life (ring phase), but develops a dedicated feeding organelle (cytostome) as it matures within a host's red blood cell.[41]

Paramecium bursaria, a ciliate which derives some of its nutrients from algal endosymbionts in the genus Chlorella

Protozoa may also live as mixotrophs, supplementing a heterotrophic diet with some form of autotrophy. Some protozoa form close associations with symbiotic photosynthetic algae, which live and grow within the membranes of the larger cell and provide nutrients to the host. Others practice kleptoplasty, stealing chloroplasts from prey organisms and maintaining them within their own cell bodies as they continue to produce nutrients through photosynthesis. The ciliate Mesodinium rubrum retains functioning plastids from the cryptophyte algae on which it feeds, using them to nourish themselves by autotrophy. These, in turn, may be passed along to dinoflagellates of the genus Dinophysis , which prey on Mesodinium rubrum but keep the enslaved plastids for themselves. Within Dinophysis, these plastids can continue to function for months.[42]

Motility[edit]

Organisms traditionally classified as protozoa are abundant in aqueous environments and soil, occupying a range of trophic levels. The group includes flagellates (which move with the help of whip-like structures called flagella), ciliates (which move by using hair-like structures called cilia) and amoebae (which move by the use of foot-like structures called pseudopodia). Some protozoa are sessile, and do not move at all.

Pellicle[edit]

Unlike plants, fungi and most types of algae, protozoans do not typically have a rigid cell wall, but are usually enveloped by elastic structures of membranes that permit movement of the cell. In some protozoans, such as the ciliates and euglenozoans, the cell is supported by a composite membranous envelope called the "pellicle". The pellicle gives some shape to the cell, especially during locomotion. Pellicles of protozoan organisms vary from flexible and elastic to fairly rigid. In ciliates and Apicomplexa, the pellicle is supported by closely packed vesicles called alveoli. In euglenids, it is formed from protein strips arranged spirally along the length of the body. Familiar examples of protists with a pellicle are the euglenoids and the ciliate Paramecium. In some protozoa, the pellicle hosts epibiotic bacteria that adhere to the surface by their fimbriae (attachment pili).[43]

Resting cyst of ciliated protozoan Dileptus viridis.

Life cycle[edit]

Life cycle of parasitic protozoan, Toxoplasma gondii

Some protozoa have two-phase life cycles, alternating between proliferative stages (e.g., trophozoites) and dormant cysts. As cysts, protozoa can survive harsh conditions, such as exposure to extreme temperatures or harmful chemicals, or long periods without access to nutrients, water, or oxygen for periods of time. Being a cyst enables parasitic species to survive outside of a host, and allows their transmission from one host to another. When protozoa are in the form of trophozoites (Greek tropho = to nourish), they actively feed. The conversion of a trophozoite to cyst form is known as encystation, while the process of transforming back into a trophozoite is known as excystation.

All protozoans reproduce (not all) asexually by binary fission or multiple fission. Many protozoan species exchange genetic material by sexual means (typically, through conjugation); however, sexuality is generally decoupled from the process of reproduction, and does not immediately result in increased population.[44]

Although meiotic sex is widespread among present day eukaryotes, it has, until recently, been unclear whether or not eukaryotes were sexual early in their evolution. Due to recent advances in gene detection and other techniques, evidence has been found for some form of meiotic sex in an increasing number of protozoans of ancient lineage that diverged early in eukaryotic evolution.[45] (See eukaryote reproduction.) Thus, such findings suggest that meiotic sex arose early in eukaryotic evolution. Examples of protozoan meiotic sexuality are described in the articles Amoebozoa, Giardia lamblia, Leishmania, Plasmodium falciparum biology, Paramecium, Toxoplasma gondii, Trichomonas vaginalis and Trypanosoma brucei.

Classification[edit]

Further information: wikispecies:Protozoa

Historically, the Protozoa were classified as "unicellular animals", as distinct from the Protophyta, single-celled photosynthetic organisms (algae) which were considered primitive plants. Both groups were commonly given the rank of phylum, under the kingdom Protista.[46] In older systems of classification, the phylum Protozoa was commonly divided into several sub-groups, reflecting the means of locomotion.[47] Classification schemes differed, but throughout much of the 20th century the major groups of Protozoa included:

- Flagellates, or Mastigophora (motile cells equipped with whiplike organelles of locomotion, e.g., Giardia lamblia)

- Amoebae or Sarcodina (cells that move by extending pseudopodia or lamellipodia, e.g., Entamoeba histolytica)

- Sporozoans, or Sporozoa (parasitic, spore-producing cells, whose adult form lacks organs of motility, e.g., Plasmodium knowlesi)

- Apicomplexa (now in Alveolata)

- Microsporidia (now in Fungi)

- Ascetosporea (now in Rhizaria)

- Myxosporidia (now in Cnidaria)

- Ciliates, or Ciliophora (cells equipped with large numbers of short hairlike organs of locomotion, e.g., Balantidium coli

With the emergence of molecular phylogenetics and tools enabling researchers to directly compare the DNA of different organisms, it became evident that, of the main sub-groups of Protozoa, only the ciliates (Ciliophora) formed a natural group, or monophyletic clade (that is, a distinct lineage of organisms sharing common ancestry). The other classes or subphyla of Protozoa were all polyphyletic groups composed of organisms that, despite similarities of appearance or way of life, were not necessarily closely related to one another. In the system of eukaryote classification currently endorsed by the International Society of Protistologists, members of the old phylum Protozoa have been distributed among a variety of supergroups.[48]

Ecology[edit]

As components of the micro- and meiofauna, protozoa are an important food source for microinvertebrates. Thus, the ecological role of protozoa in the transfer of bacterial and algal production to successive trophic levels is important. As predators, they prey upon unicellular or filamentous algae, bacteria, and microfungi. Protozoan species include both herbivores and consumers in the decomposer link of the food chain. They also control bacteria populations and biomass to some extent.

Disease[edit]

Trophozoites of the amoebic dysentery pathogen Entamoeba histolytica with ingested human red blood cells (dark circles)

Main article: Protozoan infection

A number of protozoan pathogens are human parasites, causing diseases such as malaria (by Plasmodium), amoebiasis, giardiasis, toxoplasmosis, cryptosporidiosis, trichomoniasis, Chagas disease, leishmaniasis, African trypanosomiasis (sleeping sickness), Acanthamoeba keratitis, and primary amoebic meningoencephalitis (naegleriasis).

The protozoan Ophryocystis elektroscirrha is a parasite of butterfly larvae, passed from female to caterpillar. Severely infected individuals are weak, unable to expand their wings, or unable to eclose, and have shortened lifespans, but parasite levels vary in populations. Infection creates a culling effect, whereby infected migrating animals are less likely to complete the migration. This results in populations with lower parasite loads at the end of the migration.[49] This is not the case in laboratory or commercial rearing, where after a few generations, all individuals can be infected.[50]

List of protozoan diseases in humans:[51]

Disease

Causative agent

Source of Transmission

Amoebiasis

Entamoeba histolytica (Amoebozoa)

Water, food

Acanthamoeba keratitis

Acanthamoeba (Amoebozoa)

Water, contaminated contact lens solution

Giardiasis

Giardia lamblia (Metamonada)

Water, Contact

Trichomoniasis

Trichomonas vaginalis (Metamonada)

Sexual contact

Dientamoebiasis

Dientamoeba fragilis (Metamonada)

Uncertain

African sleeping sickness (African trypanosomiasis)

Trypanosoma brucei (Kinetoplastida)

Tsetse fly (Glossina)

Chagas disease (American sleeping sickness)

Trypanosoma cruzi (Kinetoplastida)

Triatomine bug (Triatominae)

Leishmaniasis

Leishmania spp. (Kinetoplastida)

Phlebotomine Sandfly (Phlebotominae)

Balantidiasis

Balantidium coli (Ciliate)

Food, water

Malaria

Plasmodium spp. (Apicomplexa)

Mosquito (Anopheles)

Toxoplasmosis

Toxoplasma gondii (Apicomplexa)

Undercooked meat, cat feces, fetal infection in pregnancy

Babesiosis

Babesia spp. (Apicomplexa)

Deer tick (Ixodes scapularis)

Cryptosporidiosis

Cryptosporidium spp. (Apicomplexa)

Fecal contamination of food or water

Cyclosporiasis

Cyclospora cayetanensis (Apicomplexa)

Fecal contamination of food or water

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Dinoflagellate

From Wikipedia, the free encyclopedia

Dinoflagellate

Temporal range: 240–0 Ma[1]

N

Pg

K

J

T

P

C

D

S

O

Є

PreЄ

Triassic or earlier–Present

Ceratium hirundinella.jpg

Ceratium sp.

Scientific classification

Domain:

Eukaryota

(unranked):

SAR

(unranked):

Alveolata

Phylum:

Dinoflagellata

Bütschli 1885 [1880-1889] sensu Gomez 2012[2][3][4]

Classes

- Ellobiophyceae

- Psammosea

- Oxyrrhea

- Pronoctilucea

- Duboscquellea

- Syndiniophyceae

- Noctiluciphyceae

- Dinophyceae

Synonyms

- Cilioflagellata Claparède & Lachmann, 1868

- Dinophyta Dillon, 1963

- Dinophyceae sensu Pascher, 1914

- Pyrrophyta Pascher 1914

- Pyrrhophycophyta Papenfuss 1946

- Arthrodelen Flagellaten Stein 1883

- Dinomastigota Margulis & Sagan, 1985

- Dinophyta Dillon, 1963

The dinoflagellates (Greek δῖνος dinos "whirling" and Latin flagellum "whip, scourge") are single-celled eukaryotes constituting the phylum Dinoflagellata.[5] Usually considered algae, dinoflagellates are mostly marine plankton, but they also are common in freshwater habitats. Their populations are distributed depending on sea surface temperature, salinity, or depth. Many dinoflagellates are known to be photosynthetic, but a large fraction of these are in fact mixotrophic, combining photosynthesis with ingestion of prey (phagotrophy and myzocytosis).[6][7]

In terms of number of species, dinoflagellates are one of the largest groups of marine eukaryotes, although this group is substantially smaller than diatoms.[8] Some species are endosymbionts of marine animals and play an important part in the biology of coral reefs. Other dinoflagellates are unpigmented predators on other protozoa, and a few forms are parasitic (for example, Oodinium and Pfiesteria). Some dinoflagellates produce resting stages, called dinoflagellate cysts or dinocysts, as part of their lifecycles, and is known from 84 of the 350 described freshwater species, and from a little more than 10% of the known marine species.[9][10] Dinoflagellates are alveolates possessing two flagella, the ancestral condition of bikonts.

About 1,555 species of free-living marine dinoflagellates are currently described.[11] Another estimate suggests about 2,000 living species, of which more than 1,700 are marine (free-living, as well as benthic) and about 220 are from fresh water.[12] The latest estimates suggest a total of 2,294 living dinoflagellate species, which includes marine, freshwater, and parasitic dinoflagellates.[2]

A rapid accumulation of certain dinoflagellates can result in a visible coloration of the water, colloquially known as red tide (a harmful algal bloom), which can cause shellfish poisoning if humans eat contaminated shellfish. Some dinoflagellates also exhibit bioluminescence—primarily emitting blue-green light. Thus, Some parts of the Indian Ocean light up at night giving blue-green light due to bioluminescence

Contents

- 1History

- 2Morphology

- 2.1Theca structure and formation

- 2.2The dinoflagellate nucleus: dinokaryon

- 3Classification

- 3.1Identification

- 4Ecology and physiology

- 4.1Habitats

- 4.2Endosymbionts

- 4.3Nutritional strategies

- 4.4Blooms

- 4.4.1Harmful algal blooms

- 4.5Bioluminescence

- 4.6Lipid and sterol production

- 4.7Transport

- 4.8Life cycle

- 5Genomics

- 6Evolutionary history

- 7Examples

- 8See also

- 9References

- 10Bibliography

- 11External links

History[edit]

In 1753, the first modern dinoflagellates were described by Henry Baker as "Animalcules which cause the Sparkling Light in Sea Water",[13] and named by Otto Friedrich Müller in 1773.[14] The term derives from the Greek word δῖνος (dinos), meaning whirling, and Latin flagellum, a diminutive term for a whip or scourge.

In the 1830s, the German microscopist Christian Gottfried Ehrenberg examined many water and plankton samples and proposed several dinoflagellate genera that are still used today including Peridinium, Prorocentrum, and Dinophysis.[15]

These same dinoflagellates were first defined by Otto Bütschli in 1885 as the flagellate order Dinoflagellida.[16] Botanists treated them as a division of algae, named Pyrrophyta or Pyrrhophyta ("fire algae"; Greek pyrr(h)os, fire) after the bioluminescent forms, or Dinophyta. At various times, the cryptomonads, ebriids, and ellobiopsids have been included here, but only the last are now considered close relatives. Dinoflagellates have a known ability to transform from noncyst to cyst-forming strategies, which makes recreating their evolutionary history extremely difficult.

Morphology[edit]

Longitudinal (l.f.) and transverse flagellum (t.f.); sack pusule (s.p.); nucleus (n).

Dinoflagellate anatomy

Dinoflagellates are unicellular and possess two dissimilar flagella arising from the ventral cell side (dinokont flagellation). They have a ribbon-like transverse flagellum with multiple waves that beats to the cell's left, and a more conventional one, the longitudinal flagellum, that beats posteriorly.[17][18][19] The transverse flagellum is a wavy ribbon in which only the outer edge undulates from base to tip, due to the action of the axoneme which runs along it. The axonemal edge has simple hairs that can be of varying lengths. The flagellar movement produces forward propulsion and also a turning force. The longitudinal flagellum is relatively conventional in appearance, with few or no hairs. It beats with only one or two periods to its wave. The flagella lie in surface grooves: the transverse one in the cingulum and the longitudinal one in the sulcus, although its distal portion projects freely behind the cell. In dinoflagellate species with desmokont flagellation (e.g., Prorocentrum), the two flagella are differentiated as in dinokonts, but they are not associated with grooves.

Dinoflagellates have a complex cell covering called an amphiesma or cortex, composed of a series of membranes, flattened vesicles called alveolae (= amphiesmal vesicles) and related structures.[20][21] In armoured dinoflagellates, these support overlapping cellulose plates to create a sort of armor called the theca or lorica, as opposed to athecate dinoflagellates. These occur in various shapes and arrangements, depending on the species and sometimes on the stage of the dinoflagellate. Conventionally, the term tabulation has been used to refer to this arrangement of thecal plates. The plate configuration can be denoted with the plate formula or tabulation formula. Fibrous extrusomes are also found in many forms. Together with various other structural and genetic details, this organization indicates a close relationship between the dinoflagellates, the Apicomplexa, and ciliates, collectively referred to as the alveolates.[22]

Dinoflagellate tabulations can be grouped into six "tabulation types": gymnodinoid, suessoid, gonyaulacoid–peridinioid, nannoceratopsioid, dinophysioid, and prorocentroid.[citation needed]

The chloroplasts in most photosynthetic dinoflagellates are bound by three membranes, suggesting they were probably derived from some ingested algae. Most photosynthetic species contain chlorophylls a and c2, the carotenoid beta-carotene, and a group of xanthophylls that appears to be unique to dinoflagellates, typically peridinin, dinoxanthin, and diadinoxanthin. These pigments give many dinoflagellates their typical golden brown color. However, the dinoflagellates Karenia brevis, Karenia mikimotoi, and Karlodinium micrum have acquired other pigments through endosymbiosis, including fucoxanthin.[23] This suggests their chloroplasts were incorporated by several endosymbiotic events involving already colored or secondarily colorless forms. The discovery of plastids in the Apicomplexa has led some to suggest they were inherited from an ancestor common to the two groups, but none of the more basal lines has them. All the same, the dinoflagellate cell consists of the more common organelles such as rough and smooth endoplasmic reticulum, Golgi apparatus, mitochondria, lipid and starch grains, and food vacuoles. Some have even been found with a light-sensitive organelle, the eyespot or stigma, or a larger nucleus containing a prominent nucleolus. The dinoflagellate Erythropsidium has the smallest known eye.[24]

Some athecate species have an internal skeleton consisting of two star-like siliceous elements that has an unknown function, and can be found as microfossils. Tappan[25] gave a survey of dinoflagellates with internal skeletons. This included the first detailed description of the pentasters in Actiniscus pentasterias, based on scanning electron microscopy. They are placed within the order Gymnodiniales, suborder Actiniscineae.[5]

Theca structure and formation[edit]

The formation of thecal plates has been studied in detail through ultrastructural studies.[21]

The dinoflagellate nucleus: dinokaryon[edit]

'Core dinoflagellates' (dinokaryotes) have a peculiar form of nucleus, called a dinokaryon, in which the chromosomes are attached to the nuclear membrane. These carry reduced number of histones. In place of histones, dinoflagellate nuclei contain a novel, dominant family of nuclear proteins that appear to be of viral origin, thus are called dinoflagellate/ viral nucleoproteins (DVNPs) which are highly basic, bind DNA with similar affinity to histones, and occur in multiple posttranslationally modified forms.[26] Dinoflagellate nuclei remain condensed throughout interphase rather than just during mitosis, which is closed and involves a uniquely extranuclear mitotic spindle.[27] This sort of nucleus was once considered to be an intermediate between the nucleoid region of prokaryotes and the true nuclei of eukaryotes, so were termed mesokaryotic, but now are considered advanced rather than primitive traits. In addition to dinokaryotes, DVNPs can be found in a group of basal dinoflagellates (known as Marine Alveolates, "MALVs") that branch as sister to dinokaryotes (Syndiniales).[28]

Classification[edit]

Further information: Wikispecies:Dinoflagellata

1. Ornithocercus; 2. diagram; 3. Exuviaeella; 4. Prorocentrum; 5, 6. Ceratium; 7. Pouchetia; 8. Citharistes; 9. Polykrikos

Dinoflagellates are protists which have been classified using both the International Code of Botanical Nomenclature (ICBN, now renamed as ICN) and the International Code of Zoological Nomenclature (ICZN). About half of living dinoflagellate species are autotrophs possessing chloroplasts and half are nonphotosynthesising heterotrophs.

The peridinin dinoflagellates, named after their peridinin plastids, appears to be ancestral for the dinoflaggelate lineage. Almost half of all known species have chloroplasts, which are either the original peridinin plastids or new plastids acquired from other lineages of unicellular algae through endosymbiosis. The remaining species have lost their photosynthetic abilities and have adapted to a heterotrophic, parasitic or kleptoplastic lifestyle.[29][30]

Most (but not all) dinoflagellates have a dinokaryon, described below (see: Life cycle, below). Dinoflagellates with a dinokaryon are classified under Dinokaryota, while dinoflagellates without a dinokaryon are classified under Syndiniales.

Although classified as eukaryotes, the dinoflagellate nuclei are not characteristically eukaryotic, as some of them lack histones and nucleosomes, and maintain continually condensed chromosomes during mitosis. The dinoflagellate nucleus was termed ‘mesokaryotic’ by Dodge (1966),[31] due to its possession of intermediate characteristics between the coiled DNA areas of prokaryotic bacteria and the well-defined eukaryotic nucleus. This group, however, does contain typically eukaryotic organelles, such as Golgi bodies, mitochondria, and chloroplasts.[32]

Jakob Schiller (1931–1937) provided a description of all the species, both marine and freshwater, known at that time.[33] Later, Alain Sournia (1973, 1978, 1982, 1990, 1993) listed the new taxonomic entries published after Schiller (1931–1937).[34][35][36][37][38] Sournia (1986) gave descriptions and illustrations of the marine genera of dinoflagellates, excluding information at the species level.[39] The latest index is written by Gómez.[2]

Identification[edit]

English-language taxonomic monographs covering large numbers of species are published for the Gulf of Mexico,[40] the Indian Ocean,[41] the British Isles,[42] the Mediterranean[43] and the North Sea.[44]

The main source for identification of freshwater dinoflagellates is the Süsswasser Flora.[45]

Calcofluor-white can be used to stain thecal plates in armoured dinoflagellates.[46]

Ecology and physiology[edit]

Habitats[edit]

Dinoflagellates can occur in all aquatic environments: marine, brackish, and fresh water, including in snow or ice. They are also common in benthic environments and sea ice.

Endosymbionts[edit]

All Zooxanthellae are dinoflagellates and most of them are members within the genus Symbiodinium.[47] The association between Symbiodinium and reef-building corals is widely known. However, endosymbiontic Zooxanthellae inhabit a great number of other invertebrates and protists, for example many sea anemones, jellyfish, nudibranchs, the giant clam Tridacna, and several species of radiolarians and foraminiferans.[48] Many extant dinoflagellates are parasites (here defined as organisms that eat their prey from the inside, i.e. endoparasites, or that remain attached to their prey for longer periods of time, i.e. ectoparasites). They can parasitize animal or protist hosts. Protoodinium, Crepidoodinium, Piscinoodinium, and Blastodinium retain their plastids while feeding on their zooplanktonic or fish hosts. In most parasitic dinoflagellates, the infective stage resembles a typical motile dinoflagellate cell.

Nutritional strategies[edit]

Three nutritional strategies are seen in dinoflagellates: phototrophy, mixotrophy, and heterotrophy. Phototrophs can be photoautotrophs or auxotrophs. Mixotrophic dinoflagellates are photosynthetically active, but are also heterotrophic. Facultative mixotrophs, in which autotrophy or heterotrophy is sufficient for nutrition, are classified as amphitrophic. If both forms are required, the organisms are mixotrophic sensu stricto. Some free-living dinoflagellates do not have chloroplasts, but host a phototrophic endosymbiont. A few dinoflagellates may use alien chloroplasts (cleptochloroplasts), obtained from food (kleptoplasty). Some dinoflagellates may feed on other organisms as predators or parasites.[49]

Food inclusions contain bacteria, bluegreen algae, small dinoflagellates, diatoms, ciliates, and other dinoflagellates.[50][51][52][53][54][55][56]

Mechanisms of capture and ingestion in dinoflagellates are quite diverse. Several dinoflagellates, both thecate (e.g. Ceratium hirundinella,[55] Peridinium globulus[53]) and nonthecate (e.g. Oxyrrhis marina,[51] Gymnodinium sp.[57] and Kofoidinium spp.[58]), draw prey to the sulcal region of the cell (either via water currents set up by the flagella or via pseudopodial extensions) and ingest the prey through the sulcus. In several Protoperidinium spp., e.g. P. conicum, a large feeding veil — a pseudopod called the pallium — is extruded to capture prey which is subsequently digested extracellularly (= pallium-feeding).[59][60] Oblea, Zygabikodinium, and Diplopsalis are the only other dinoflagellate genera known to use this particular feeding mechanism.[60][61][62] Katodinium (Gymnodinium) fungiforme, commonly found as a contaminant in algal or ciliate cultures, feeds by attaching to its prey and ingesting prey cytoplasm through an extensible peduncle.[63] Two related species, polykrikos kofoidii and neatodinium, shoots out a harpoon-like organelle to capture prey.[64] The feeding mechanisms of the oceanic dinoflagellates remain unknown, although pseudopodial extensions were observed in Podolampas bipes.[65]

Blooms[edit]

Dinoflagellate blooms are generally unpredictable, short, with low species diversity, and with little species succession.[66] The low species diversity can be due to multiple factors. One way a lack of diversity may occur in a bloom is through a reduction in predation and a decreased competition. The first may be achieved by having predators reject the dinoflagellate, by, for example, decreasing the amount of food it can eat. This additionally helps prevent a future increase in predation pressure by cause predators that reject it to lack the energy to breed. A species can then inhibit the growth of its competitors, thus achieving dominance.[67]

Harmful algal blooms[edit]

Dinoflagellates sometimes bloom in concentrations of more than a million cells per millilitre. Under such circumstances, they can produce toxins (generally called dinotoxins) in quantities capable of killing fish and accumulating in filter feeders such as shellfish, which in turn may be passed on to people who eat them. This phenomenon is called a red tide, from the color the bloom imparts to the water. Some colorless dinoflagellates may also form toxic blooms, such as Pfiesteria. Some dinoflagellate blooms are not dangerous. Bluish flickers visible in ocean water at night often come from blooms of bioluminescent dinoflagellates, which emit short flashes of light when disturbed.

Algal bloom (akasio) by Noctiluca spp. in Nagasaki

The same red tide mentioned above is more specifically produced when dinoflagellates are able to reproduce rapidly and copiously on account of the abundant nutrients in the water. Although the resulting red waves are an unusual sight, they contain toxins that not only affect all marine life in the ocean, but the people who consume them, as well.[68] A specific carrier is shellfish. This can introduce both nonfatal and fatal illnesses. One such poison is saxitoxin, a powerful paralytic neurotoxin. Human inputs of phosphate further encourage these red tides, so strong interest exists in learning more about dinoflagellates, from both medical and economic perspectives. Dinoflagellates are known to be particularly capable of scavenging dissolved organic phosphorus for P-nutrient, several HAS species have been found to be highly versatile and mechanistically diversified in utilizing different types of DOPs.[69] [70] [71] The ecology of harmful algal blooms is extensively studied.[72]

Bioluminescence[edit]

Long exposure image of bioluminescence of N. scintillans in the yacht port of Zeebrugge, Belgium

Kayaking in the Bioluminescent Bay, Vieques, Puerto Rico

At night, water can have an appearance of sparkling light due to the bioluminescence of dinoflagellates.[73][74] More than 18 genera of dinoflagellates are bioluminescent,[75] and the majority of them emit a blue-green light.[76] These species contain scintillons, individual cytoplasmic bodies (about 0.5 µm in diameter) distributed mainly in the cortical region of the cell, outpockets of the main cell vacuole. They contain dinoflagellate luciferase, the main enzyme involved in dinoflagellate bioluminescence, and luciferin, a chlorophyll-derived tetrapyrrole ring that acts as the substrate to the light-producing reaction. The luminescence occurs as a brief (0.1 sec) blue flash (max 476 nm) when stimulated, usually by mechanical disturbance. Therefore, when mechanically stimulated—by boat, swimming, or waves, for example—a blue sparkling light can be seen emanating from the sea surface.[77]

Dinoflagellate bioluminescence is controlled by a circadian clock and only occurs at night.[78] Luminescent and nonluminescent strains can occur in the same species. The number of scintillons is higher during night than during day, and breaks down during the end of the night, at the time of maximal bioluminescence.[79]

The luciferin-luciferase reaction responsible for the bioluminescence is pH sensitive.[77] When the pH drops, luciferase changes its shape, allowing luciferin, more specifically tetrapyrrole, to bind.[77] Dinoflagellates can use bioluminescence as a defense mechanism. They can startle their predators by their flashing light or they can ward off potential predators by an indirect effect such as the "burglar alarm". The bioluminescence attracts attention to the dinoflagellate and its attacker, making the predator more vulnerable to predation from higher trophic levels.[77]

Bioluminescent dinoflagellate ecosystem bays are among the rarest and most fragile,[80] with the most famous ones being the Bioluminescent Bay in La Parguera, Lajas, Puerto Rico; Mosquito Bay in Vieques, Puerto Rico; and Las Cabezas de San Juan Reserva Natural Fajardo, Puerto Rico. Also, a bioluminescent lagoon is near Montego Bay, Jamaica, and bioluminescent harbors surround Castine, Maine.[81] Within the United States, Central Florida is home to the Indian River Lagoon which is abundant with dinoflagellates in the summer and bioluminescent ctenophore in the winter.[82]

Lipid and sterol production[edit]

Dinoflagellates produce characteristic lipids and sterols.[83] One of these sterols is typical of dinoflagellates and is called dinosterol.

Transport[edit]

Dinoflagellate theca can sink rapidly to the seafloor in marine snow.[84]

Life cycle[edit]

Dinoflagellata life cycle: 1-mitosis, 2-sexual reproduction, 3-planozygote, 4-hypnozygote, 5-planomeiocyte

Dinoflagellates have a haplontic life cycle, with the possible exception of Noctiluca and its relatives.[5] The life cycle usually involves asexual reproduction by means of mitosis, either through desmoschisis or eleuteroschisis. More complex life cycles occur, more particularly with parasitic dinoflagellates. Sexual reproduction also occurs,[85] though this mode of reproduction is only known in a small percentage of dinoflagellates.[86] This takes place by fusion of two individuals to form a zygote, which may remain mobile in typical dinoflagellate fashion and is then called a planozygote. This zygote may later form a resting stage or hypnozygote, which is called a dinoflagellate cyst or dinocyst. After (or before) germination of the cyst, the hatchling undergoes meiosis to produce new haploid cells. Dinoflagellates appear to be capable of carrying out several DNA repair processes that can deal with different types of DNA damage[87]

Genomics[edit]

One of their most striking features is the large amount of cellular DNA that dinoflagellates contain. Most eukaryotic algae contain on average about 0.54 pg DNA/cell, whereas estimates of dinoflagellate DNA content range from 3–250 pg/cell,[27] corresponding to roughly 3000–215 000 Mb (in comparison, the haploid human genome is 3180 Mb and hexaploid Triticum wheat is 16 000 Mb). Polyploidy or polyteny may account for this large cellular DNA content,[88] but earlier studies of DNA reassociation kinetics and recent genome analyses do not support this hypothesis.[89] Rather, this has been attributed, hypothetically, to the rampant retroposition found in dinoflagellate genomes.[90] [91]

In addition to their disproportionately large genomes, dinoflagellate nuclei are unique in their morphology, regulation, and composition. Their DNA is so tightly packed it is still uncertain exactly how many chromosomes they have.[92]

The dinoflagellates share an unusual mitochondrial genome organisation with their relatives, the Apicomplexa.[93] Both groups have very reduced mitochondrial genomes (around 6 kilobases (kb) in the Apicomplexa vs ~16kb for human mitochondria). One species, Amoebophrya ceratii, has lost its mitochondrial genome completely, yet still has functional mitochondria.[94] The genes on the dinoflagellate genomes have undergone a number of reorganisations, including massive genome amplification and recombination which have resulted in multiple copies of each gene and gene fragments linked in numerous combinations. Loss of the standard stop codons, trans-splicing of mRNAs for the mRNA of cox3, and extensive RNA editing recoding of most genes has occurred.[95] [96] The reasons for this transformation are unknown. In a small group of dinoflagellates, called ‘dinotoms’ (Durinskia and Kryptoperidinium), the endosymbionts (diatoms) still have mitochondria, making them the only organisms with two evolutionarily distinct mitochondria.[97]

In most of the species, the plastid genome consist of just 14 genes.[98]

The DNA of the plastid in the peridinin-containing dinoflagellates is contained in a series of small circles.[99] Each circle contains one or two polypeptide genes. The genes for these polypeptides are chloroplast-specific because their homologs from other photosynthetic eukaryotes are exclusively encoded in the chloroplast genome. Within each circle is a distinguishable 'core' region. Genes are always in the same orientation with respect to this core region.

In terms of DNA barcoding, ITS sequences can be used to identify species,[100] where a genetic distance of p≥0.04 can be used to delimit species,[101] which has been successfully applied to resolve long-standing taxonomic confusion as in the case of resolving the Alexandrium tamarense complex into five species.[102] A recent study[103] revealed a substantial proportion of dinoflagellate genes encode for unknown functions, and that these genes could be conserved and lineage-specific.

Evolutionary history[edit]

Dinoflagellates are mainly represented as fossils by fossil dinocysts, which have a long geological record with lowest occurrences during the mid-Triassic,[104] whilst geochemical markers suggest a presence to the Early Cambrian.[105]

Some evidence indicates dinosteroids in many Paleozoic and Precambrian rocks might be the product of ancestral dinoflagellates (protodinoflagellates).[106][107]

Molecular phylogenetics show that dinoflagellates are grouped with ciliates and apicomplexans (=Sporozoa) in a well-supported clade, the alveolates. The closest relatives to dinokaryotic dinoflagellates appear to be apicomplexans, Perkinsus, Parvilucifera, syndinians, and Oxyrrhis.[108] Molecular phylogenies are similar to phylogenies based on morphology.[109]

The earliest stages of dinoflagellate evolution appear to be dominated by parasitic lineages, such as perkinsids and syndinians (e.g. Amoebophrya and Hematodinium).[110][111][112][113]

All dinoflagellates contain red algal plastids or remnant (nonphotosynthetic) organelles of red algal origin.[114] The parasitic dinoflagellate Hematodinium however lacks a plastid entirely.[115] Some groups that have lost the photosynthetic properties of their original red algae plastids has obtained new photosynthetic plastids (chloroplasts) through so-called serial endosymbiosis, both secondary and tertiary. Like their original plastids, the new chloroplasts in these groups can be traced back to red algae, except from those in the members of the genus Lepidodinium, which possess plastids derived from green algae, possibly Trebouxiophyceae or Ulvophyceae.[116][117] Lineages with tertiary endosymbiosis are Dinophysis, with plastids from a cryptomonad,[118] the Karenia, Karlodinium, and Takayama, which possess plastids of haptophyte origin, and the Peridiniaceae, Durinskia and Kryptoperidinium, which has plastids derived from diatoms[119][120] Some species also perform kleptoplasty.[121]

Dinoflagellate evolution has been summarized into five principal organizational types: prorocentroid, dinophysoid, gonyaulacoid, peridinioid, and gymnodinoid.[122] The transitions of marine species into fresh water have been infrequent events during the diversification of dinoflagellates and in most cases have not occurred recently, possibly as late as the Cretaceous.[123]

Recently, the "living fossil" Dapsilidinium pastielsii was found inhabiting the Indo-Pacific Warm Pool, which served as a refugium for thermophilic dinoflagellates.[124]

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Amoeba

From Wikipedia, the free encyclopedia

(Redirected from Amoebae)

This article is about the cellular body type. For the genus, see Amoeba (genus). For other uses, see Amoeba (disambiguation).

Clockwise from top right: Amoeba proteus, Actinophrys sol, Acanthamoeba sp., Pompholyxophrys sp., Euglypha sp., neutrophil ingesting bacteria

An amoeba or ameba (/əˈmiːbə/; rarely spelt amœba; plural am(o)ebas or am(o)ebae /əˈmiːbi/),[1] often called an amoeboid, is a type of cell or unicellular organism which has the ability to alter its shape, primarily by extending and retracting pseudopods.[2] Amoebae do not form a single taxonomic group; instead, they are found in every major lineage of eukaryotic organisms. Amoeboid cells occur not only among the protozoa, but also in fungi, algae, and animals.[3][4][5][6][7]

Microbiologists often use the terms "amoeboid" and "amoeba" interchangeably for any organism that exhibits amoeboid movement.[8][9]

In older classification systems, most amoebae were placed in the class or subphylum Sarcodina, a grouping of single-celled organisms that possess pseudopods or move by protoplasmic flow. However, molecular phylogenetic studies have shown that Sarcodina is not a monophyletic group whose members share common descent. Consequently, amoeboid organisms are no longer classified together in one group.[10]

The best known amoeboid protists are the "giant amoebae" Chaos carolinense and Amoeba proteus, both of which have been widely cultivated and studied in classrooms and laboratories.[11][12] Other well known species include the so-called "brain-eating amoeba" Naegleria fowleri, the intestinal parasite Entamoeba histolytica, which causes amoebic dysentery, and the multicellular "social amoeba" or slime mould Dictyostelium discoideum.

Contents

- 1Shape, movement and nutrition

- 2Size range

- 3Amoebae as specialized cells and life cycle stages

- 4Amoebae as organisms

- 4.1Early history and origins of Sarcodina

- 4.2Dismantling of Sarcodina

- 4.3Classification

- 5Pathogenic interactions with other organisms

- 6Meiosis

- 7References

- 8External links

Shape, movement and nutrition[edit]

Amoebae do not have cell walls, which allows for free movement. Amoebae move and feed by using pseudopods, which are bulges of cytoplasm formed by the coordinated action of actin microfilaments pushing out the plasma membrane that surrounds the cell.[13]

The forms of pseudopodia, from left: polypodial and lobose; monopodial and lobose; filose; conical; reticulose; tapering actinopods; non-tapering actinopods

The appearance and internal structure of pseudopods are used to distinguish groups of amoebae from one another. Amoebozoan species, such as those in the genus Amoeba, typically have bulbous (lobose) pseudopods, rounded at the ends and roughly tubular in cross-section. Cercozoan amoeboids, such as Euglypha and Gromia, have slender, thread-like (filose) pseudopods. Foraminifera emit fine, branching pseudopods that merge with one another to form net-like (reticulose) structures. Some groups, such as the Radiolaria and Heliozoa, have stiff, needle-like, radiating axopodia (actinopoda) supported from within by bundles of microtubules.[3][14]

Morphology of a naked lobose amoeba

Free-living amoebae may be "testate" (enclosed within a hard shell), or "naked" (also known as gymnamoebae, lacking any hard covering). The shells of testate amoebae may be composed of various substances, including calcium, silica, chitin, or agglutinations of found materials like small grains of sand and the frustules of diatoms.[15]

Shell of the testate amoeba Difflugia acuminata.

To regulate osmotic pressure, most freshwater amoebae have a contractile vacuole which expels excess water from the cell.[16] This organelle is necessary because freshwater has a lower concentration of solutes (such as salt) than the amoeba's own internal fluids (cytosol). Because the surrounding water is hypotonic with respect to the contents of the cell, water is transferred across the amoeba's cell membrane by osmosis. Without a contractile vacuole, the cell would fill with excess water and, eventually, burst.

Marine amoebae do not usually possess a contractile vacuole because the concentration of solutes within the cell are in balance with the tonicity of the surrounding water.[17]

Amoeba phagocytosis of bacteria

The food sources of amoebae vary. Some amoebae are predatory and live by consuming bacteria and other protists. Some are detritivores and eat dead organic material.

Amoebae typically ingest their food by phagocytosis, extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth or cytostome, and there is no fixed place on the cell at which phagocytosis normally occurs.[18]

Some amoebae also feed by pinocytosis, imbibing dissolved nutrients through vesicles formed within the cell membrane .[19]

Size range[edit]

Foraminifera have reticulose (net-like) pseudopods, and many species are visible with naked eye

The size of amoeboid cells and species is extremely variable. The marine amoeboid Massisteria voersi is just 2.3 to 3 micrometres in diameter,[20] within the size range of many bacteria.[21] At the other extreme, the shells of deep-sea xenophyophores can attain 20 cm in diameter.[22] Most of the free-living freshwater amoebae commonly found in pond water, ditches, and lakes are microscopic, but some species, such as the so-called "giant amoebae" Pelomyxa palustris and Chaos carolinense, can be large enough to see with the naked eye.

Species or cell type

Size in micrometres

Massisteria voersi[20]

2.3–3

Naegleria fowleri[23]

8–15

Neutrophil (white blood cell)[24]

12–15

Acanthamoeba[25]

12–40

Entamoeba histolytica[26]

15–60

Arcella vulgaris[27]

30–152

Amoeba proteus[28]

220–760

Chaos carolinense[29]

700–2000

Pelomyxa palustris[30]

up to 5000

Syringammina fragilissima[31]

up to 200000

Amoebae as specialized cells and life cycle stages[edit]

Neutrophil (white blood cell) engulfing anthrax bacteria

Some multicellular organisms have amoeboid cells only in certain phases of life, or use amoeboid movements for specialized functions. In the immune system of humans and other animals, amoeboid white blood cells pursue invading organisms, such as bacteria and pathogenic protists, and engulf them by phagocytosis.[32]

Amoeboid stages also occur in the multicellular fungus-like protists, the so-called slime moulds. Both the plasmodial slime moulds, currently classified in the class Myxogastria, and the cellular slime moulds of the groups Acrasida and Dictyosteliida, live as amoebae during their feeding stage. The amoeboid cells of the former combine to form a giant multinucleate organism,[33] while the cells of the latter live separately until food runs out, at which time the amoebae aggregate to form a multicellular migrating "slug" which functions as a single organism.[8]

Other organisms may also present amoeboid cells during certain life-cycle stages, e.g., the gametes of some green algae (Zygnematophyceae)[34] and pennate diatoms,[35] the spores (or dispersal phases) of some Mesomycetozoea,[36][37] and the sporoplasm stage of Myxozoa and of Ascetosporea.[38]

Amoebae as organisms[edit]

Early history and origins of Sarcodina[edit]

The first illustration of an amoeboid, from Roesel von Rosenhof's Insecten-Belustigung (1755).

The earliest record of an amoeboid organism was produced in 1755 by August Johann Rösel von Rosenhof, who named his discovery "Der Kleine Proteus" ("the Little Proteus").[39] Rösel's illustrations show an unidentifiable freshwater amoeba, similar in appearance to the common species now known as Amoeba proteus.[40] The term "Proteus animalcule" remained in use throughout the 18th and 19th centuries, as an informal name for any large, free-living amoeboid.[41]

In 1822, the genus Amiba (from the Greek ἀμοιβή amoibe, meaning "change") was erected by the French naturalist Bory de Saint-Vincent.[42][43] Bory's contemporary, C. G. Ehrenberg, adopted the genus in his own classification of microscopic creatures, but changed the spelling to Amoeba.[44]

In 1841, Félix Dujardin coined the term "sarcode" (from Greek σάρξ sarx, "flesh," and εἶδος eidos, "form") for the "thick, glutinous, homogenous substance" which fills protozoan cell bodies.[45] Although the term originally referred to the protoplasm of any protozoan, it soon came to be used in a restricted sense to designate the gelatinous contents of amoeboid cells.[10] Thirty years later, the Austrian zoologist Ludwig Karl Schmarda used "sarcode" as the conceptual basis for his Division Sarcodea, a phylum-level group made up of "unstable, changeable" organisms with bodies largely composed of 'sarcode.'[46] Later workers, including the influential taxonomist Otto Bütschli, emended this group to create the class Sarcodina,[47] a taxon that remained in wide use throughout most of the 20th century.

Within the traditional Sarcodina, amoebae were generally divided into morphological categories, on the basis of the form and structure of their pseudopods. Amoebae with pseudopods supported by regular arrays of microtubules (such as the freshwater Heliozoa and marine Radiolaria) were classified as Actinopods; whereas those with unsupported pseudopods were classified as Rhizopods.[48] The Rhizopods were further subdivided into lobose, filose, and reticulose amoebae, according to the morphology of their pseudopods.

Dismantling of Sarcodina[edit]

In the final decade of the 20th century, a series of molecular phylogenetic analyses confirmed that Sarcodina was not a monophyletic group. In view of these findings, the old scheme was abandoned and the amoebae of Sarcodina were dispersed among many other high-level taxonomic groups. Today, the majority of traditional sarcodines are placed in two eukaryote supergroups: Amoebozoa and Rhizaria. The rest have been distributed among the excavates, opisthokonts, and stramenopiles. Some, like the Centrohelida, have yet to be placed in any supergroup.[10][49]

Classification[edit]

Recent classification places the various amoeboid genera in the following groups:

Supergroups

Major groups and genera

Morphology

Amoebozoa

- Lobosa:

- Acanthamoeba, Amoeba, Balamuthia, Chaos, Clydonella, Discamoeba, Echinamoeba, Filamoeba, Flabellula, Gephyramoeba, Glaeseria, Hartmannella, Hydramoeba, Korotnevella (Dactylamoeba), Leptomyxa, Lingulamoeba, Mastigina, Mayorella, Metachaos, Neoparamoeba, Paramoeba, Polychaos, Phreatamoeba, Platyamoeba, Protoacanthamoeba, Rhizamoeba, Saccamoeba, Sappinia, Stereomyxa, Thecamoeba, Trichamoeba, Trichosphaerium, Unda, Vannella, Vexillifera

- Conosa: Endamoeba, Entamoeba, Hyperamoeba, Mastigamoeba, Mastigella, Pelomyxa, Dictyostelium, Physarum

- Lobose pseudopods (Lobosa): Lobose pseudopods are blunt, and there may be one or several on a cell, which is usually divided into a layer of clear ectoplasm surrounding more granular endoplasm.

Rhizaria

- Cercozoa:

- Filosa:

- Monadofilosa: Gyromitus, Paulinella

- Granofilosea

- Chlorarachniophyceae

- Endomyxa:

- Proteomyxidea: Vampyrella

- Gromiidea

- Foraminifera

- Radiolaria

- Filose pseudopods (Filosa): Filose pseudopods are narrow and tapering. The vast majority of filose amoebae, including all those that produce shells, are placed within the Cercozoa together with various flagellates that tend to have amoeboid forms. The naked filose amoebae also includes vampyrellids.

- Reticulose pseudopods (Endomyxa): Reticulose pseudopods are cytoplasmic strands that branch and merge to form a net. They are found most notably among the Foraminifera, a large group of marine protists that generally produce multi-chambered shells. There are only a few sorts of naked reticulose amoebae, notably the gymnophryids, and their relationships are not certain.

- Radiolarians are a subgroup of actinopods that are now grouped with rhizarians.

Excavata

- Heterolobosea:

- Vahlkampfiidae: Monopylocystis, Naegleria, Neovahlkampfia, Paratetramitus, Paravahlkampfia, Psalteriomonas, Sawyeria, Tetramitus, Vahlkampfia, Willaertia

- Gruberellidae: Gruberella, Stachyamoeba

- Parabasalidea: Dientamoeba, Histomonas

- Other: Rosculus, Acrasis, Heteramoeba, Learamoeba, Stygamoeba, Plaesiobystra,[50] Tulamoeba[50]

- The Heterolobosea, includes protists that can transform between amoeboid and flagellate forms.

Heterokonta

- Chrysophyceae: Chrysamoeba, Rhizochrysis

- Xanthophyceae: Rhizochloris

- Labyrinthulomycetes

- The heterokont chrysophyte and xanthophyte algae includes some amoeboid members, the latter being poorly studied.[51]

Alveolata

- Dinoflagellata: Dinamoeba, Pfiesteria

- Parasite with amoeboid life cycle stages.

Opisthokonta

- Nucleariida: Micronuclearia, Nuclearia

- Nucleariids appear to be close relatives of animals and fungi.

Ungrouped/

unknown

- Adelphamoeba, Astramoeba, Cashia, Dinamoeba, Flagellipodium, Flamella, Gibbodiscus, Gocevia, Hollandella, Iodamoeba, Malamoeba, Nollandia, Oscillosignum, Paragocevia, Parvamoeba, Pernina, Pontifex, Protonaegleria, Pseudomastigamoeba, Rugipes, Striamoeba, Striolatus, Subulamoeba, Theratromyxa, Trienamoeba, Trimastigamoeba, Vampyrellium, and about 50 other genera[52]

Some of the amoeboid groups cited (e.g., part of chrysophytes, part of xanthophytes, chlorarachniophytes) were not traditionally included in Sarcodina, being classified as algae or flagellated protozoa.

Pathogenic interactions with other organisms[edit]

Trophozoites of the pathogenic Entamoeba histolytica with ingested red blood cells

Some amoebae can infect other organisms pathogenically, causing disease:

- Entamoeba histolytica is the cause of amoebiasis, or amoebic dysentery.

- Naegleria fowleri (the "brain-eating amoeba") is a fresh-water-native species that can be fatal to humans if introduced through the nose.

- Acanthamoeba can cause amoebic keratitis and encephalitis in humans.

- Balamuthia mandrillaris is the cause of (often fatal) granulomatous amoebic meningoencephalitis

- Amoeba have been found to harvest and grow the bacteria implicated in plague.

- Amoebae can likewise play host to microscopic organisms that are pathogenic to people and help in spreading such microbes. Bacterial pathogens, for example, Legionella can oppose absorption of food when devoured by amoebae. . [53]

- The presently generally utilized and best-explored amoebae which host other organisms are Acanthamoeba castellanii and Dictyostelium discoideum.[54]

- Microorganisms that can overcome one-celled critters' guards increase a shelter wherein to multiply, where they are shielded from unfriendly outside conditions by their accidental hosts.

[55][56] [57] [58]

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Amoebozoa

From Wikipedia, the free encyclopedia

Amoebozoa

Temporal range: 1000–0 Ma[1]

Had'n

Archean

Proterozoic

Pha.

Chaos carolinense.jpg

Chaos carolinensis

Scientific classification

Domain:

Eukaryota

(unranked):

Unikonta

(unranked):

Amoebozoa

Lühe, 1913 emend. Cavalier-Smith, 1998

Subphyla, Infraphyla and Classes

- Discosea

- Centramoebia

- Flabellinia

- Tubulinea

- Corycidia

- Echinamoebia

- Elardia

- Evosida

- Cutosa

- Cutosea

- Conosa

- Archamoebae

- Archamoebea

- Semiconosa

- Variosea (paraphyletic)

- Dictyostelea

- Ceratiomyxomycetes

- Myxomycetes

Synonyms

- Eumycetozoa Zopf 1884, emend Olive 1975

Amoebozoa is a major taxonomic group containing about 2,400 described species of amoeboid protists,[2] often possessing blunt, fingerlike, lobose pseudopods and tubular mitochondrial cristae.[3][4] In most classification schemes, Amoebozoa is ranked as a phylum within either the kingdom Protista[5] or the kingdom Protozoa.[6] In the classification favored by the International Society of Protistologists, it is retained as an unranked "supergroup" within Eukaryota.[3] Molecular genetic analysis supports Amoebozoa as a monophyletic clade. Most phylogenetic trees identify it as the sister group to Opisthokonta, another major clade which contains both fungi and animals as well as some 300 species of unicellular protists.[2][4] Amoebozoa and Opisthokonta are sometimes grouped together in a high-level taxon, variously named Unikonta,[6] Amorphea[3] or Opimoda.[7]

Amoebozoa includes many of the best-known amoeboid organisms, such as Chaos, Entamoeba, Pelomyxa and the genus Amoeba itself. Species of Amoebozoa may be either shelled (testate), or naked, and cells may possess flagella. Free-living species are common in both salt and freshwater as well as soil, moss and leaf litter. Some live as parasites or symbiotes of other organisms, and some are known to cause disease in humans and other organisms.

While the majority of amoebozoan species are unicellular, the group also includes several varieties of slime molds, which have a macroscopic, multicellular stage of life during which individual amoeboid cells aggregate to produce spores.

Amoebozoa vary greatly in size. Some are only 10–20 μm in diameter, while others are among the largest protozoa. The well-known species Amoeba proteus, which may reach 800 μm in length, is often studied in schools and laboratories as a representative cell or model organism, partly because of its convenient size. Multinucleate amoebae like Chaos and Pelomyxa (the so-called "giant amoebae") may be several millimetres in length, and some multicellular amoebozoa, such as the "dog vomit" slime mold Fuligo septica, can cover an area of several square meters.[8]

Contents

- 1Morphology

- 2Classification

- 2.1Place of Amoebozoa in the eukaryote tree

- 2.2Subphyla within Amoebozoa: Lobosa and Conosa

- 2.3Phylogeny and taxonomy within Amoebozoa

- 3Fossil record

- 4List of amoebozoan protozoa pathogenic to humans

- 5Meiosis

- 6Human health

- 7Gallery

- 8References

- 9Further reading

- 10External links

Morphology[edit]

An amoeba of the genus Mayorella (Amoebozoa, Discosea)

Amoebozoa is a large and diverse group, but certain features are common to many of its members. The amoebozoan cell is typically divided into a granular central mass, called endoplasm, and a clear outer layer, called ectoplasm. During locomotion, the endoplasm flows forwards and the ectoplasm runs backwards along the outside of the cell. In motion, many amoebozoans have a clearly defined anterior and posterior and may assume a "monopodial" form, with the entire cell functioning as a single pseudopod. Large pseudopods may produce numerous clear projections called subpseudopodia (or determinate pseudopodia), which are extended to a certain length and then retracted, either for the purpose of locomotion or food intake. A cell may also form multiple indeterminate pseudopodia, through which the entire contents of the cell flow in the direction of locomotion. These are more or less tubular and are mostly filled with granular endoplasm. The cell mass flows into a leading pseudopod, and the others ultimately retract, unless the organism changes direction.[9]

While most amoebozoans are "naked," like the familiar Amoeba and Chaos, or covered with a loose coat of minute scales, like Cochliopodium and Korotnevella, members of the order Arcellinida form rigid shells, or tests, equipped with a single aperture through which the pseudopods emerge. Arcellinid tests may be secreted from organic materials, as in Arcella, or built up from collected particles cemented together, as in Difflugia.

In all amoebozoa, the primary mode of nutrition is phagocytosis, in which the cell surrounds potential food particles with its pseudopods, sealing them into vacuoles within which they may be digested and absorbed. Some amoebozoans have a posterior bulb called a uroid, which may serve to accumulate waste, periodically detaching from the rest of the cell.[citation needed] When food is scarce, most species can form cysts, which may be carried aerially and introduce them to new environments.[citation needed] In slime moulds, these structures are called spores, and form on stalked structures called fruiting bodies or sporangia.

The majority of Amoebozoa lack flagella and more generally do not form microtubule-supported structures except during mitosis. However, flagella do occur among the Archamoebae, and many slime moulds produce biflagellate gametes[citation needed]. The flagellum is generally anchored by a cone of microtubules, suggesting a close relationship to the opisthokonts.[citation needed] The mitochondria in amoebozoan cells characteristically have branching tubular cristae. However, among the Archamoebae, which are adapted to anoxic or microaerophilic habitats, mitochondria have been lost.

Classification[edit]

Further information: wikispecies:Amoebozoa

Place of Amoebozoa in the eukaryote tree[edit]

It appears (based on molecular genetics) that the members of Amoebozoa form a sister group to animals and fungi, diverging from this lineage after it had split from the other groups,[10] as illustrated below in a simplified diagram:

Opimoda

Loukozoa

Giardia lamblia.jpg

Podiata

CRuMs

Collodictyon anterior view, showing sulcus, nucleus, blepharoplast, rhizoplast, and four flagella..jpg

Amorphea

Amoebozoa

Obazoa

Breviata

Mastigamoeba invertens.jpg

Apusomonadida

Apusomonas.png

Opisthokonta

Fungi

Asco1013.jpg

Animalia

Comb jelly.jpg

Strong similarities between Amoebozoa and Opisthokonts lead to the hypothesis that they form a distinct clade.[11] Thomas Cavalier-Smith proposed the name "unikonts" (formally, Unikonta) for this branch, whose members were believed to have been descended from a common ancestor possessing a single emergent flagellum rooted in one basal body.[1][2] However, while the close relationship between Amoebozoa and Opisthokonta is robustly supported, recent work has shown that the hypothesis of a uniciliate ancestor is probably false. In their Revised Classification of Eukaryotes (2012), Adl et al. proposed Amorphea as a more suitable name for a clade of approximately the same composition, a sister group to the Diaphoretickes.[3] More recent work places the members of Amorphea together with the malawimonids and collodictyonids in a proposed clade called Opimoda, which comprises one of two major lineages diverging at the root of the eukaryote tree of life.[7]

See also: Eukaryote § Phylogeny

Subphyla within Amoebozoa: Lobosa and Conosa[edit]

Traditionally all amoebozoa with lobose pseudopods were grouped together in the class Lobosea, placed with other amoeboids in the phylum Sarcodina or Rhizopoda, but these were considered to be unnatural groups. Structural and genetic studies identified the percolozoans and several archamoebae as independent groups. In phylogenies based on rRNA their representatives were separate from other amoebae, and appeared to diverge near the base of eukaryotic evolution, as did most slime molds.

However, revised trees by Cavalier-Smith and Chao in 1996[12] suggested that the remaining lobosans do form a monophyletic group, to which the Archamoebae and Mycetozoa were closely related, although the percolozoans were not. Subsequently, they emended the phylum Amoebozoa to include both the subphylum Lobosa and a new subphylum Conosa, comprising the Archamoebae and the Mycetozoa.[13]

Recent molecular genetic data appear to support this primary division of the Amoebozoa into Lobosa and Conosa.[4] The former, as defined by Cavalier-Smith and his collaborators, consists largely of the classic Lobosea: non-flagellated amoebae with blunt, lobose pseudopods (Amoeba, Acanthamoeba, Arcella, Difflugia etc.). The latter is made up of both amoeboid and flagellated cells, characteristically with more pointed or slightly branching subpseudopodia (Archamoebae and the Mycetozoan slime molds).

Phylogeny and taxonomy within Amoebozoa[edit]

From older studies by Cavalier-Smith, Chao & Lewis 2016[14] and Silar 2016.[15] Also recent phylogeny indicates the Lobosa are paraphyletic: Conosa is sister of the Cutosea.[16][17][18]

showAmoebozoa phylogeny

Phylum Amoebozoa Lühe 1913 emend. Cavalier-Smith 1998 [Amoebobiota; Eumycetozoa Zopf 1884 emend Olive 1975]

- Clade Discosea Cavalier-Smith 2004 stat. nov. Adl et al. 2018

- Order ?Stereomyxida Grell 1971

- Order ?Stygamoebida Smirnov & Cavalier-Smith 2011

- Class Centramoebia Cavalier-Smith et al. 2016

- Order Centramoebida Rogerson & Patterson 2002 em. Cavalier-Smith 2004

- Order Himatismenida Page 1987 [Cochliopodiida]

- Order Pellitida Page 1987 [Cochliopodiida]

- Class Flabellinia Smirnov & Cavalier-Smith 2011 em. Kudryavtsev et al. 2014

- Order Thecamoebida Schaeffer 1926 em. Smirnov & Cavalier-Smith 2011

- Order Dermamoebida Cavalier-Smith 2004 em. Smirnov & Cavalier-Smith 2011

- Order Vannellida Smirnov et al. 2005

- Order Dactylopodida Smirnov et al. 2005

- Clade Tevosa Kang et al. 2017

- Clade Tubulinea Smirnov et al. 2005 stat. nov. Adl et al. 2018

- Class Corycidia Kang et al. 2017 stat. nov. Adl et al. 2018

- Order Trichosida Moebius 1889

- Family Microcoryciidae de Saedeleer 1934

- Class Echinamoebia Cavalier-Smith 2016 stat. nov. Adl et al. 2018

- Order Echinamoebida Cavalier-Smith 2004 em. 2011

- Class Elardia Kang et al. 2017 stat. nov. Adl et al. 2018

- Subclass Leptomyxia Cavalier-Smith 2016

- Order Leptomyxida Pussard & Pons 1976 em. Page 1987

- Subclass Eulobosia Cavalier-Smith 2016

- Order Euamoebida Lepşi 1960 em. Cavalier-Smith 2016

- Order Arcellinida Kent 1880

- Clade Evosea Kang et al. 2017 stat. nov. Adl et al. 2018

- Clade Cutosa Cavalier-Smith 2016 stat. nov.

- Class Cutosea Cavalier-Smith 2016

- Order Squamocutida Cavalier-Smith 2016

- Subphylum Conosa Cavalier-Smith 1998 stat. nov.

- Infraphylum Archamoebae Cavalier-Smith 1993 stat. n. 1998

- Class Archamoebea Cavalier-Smith 1983 stat. n. 2004

- Family Tricholimacidae Cavalier-Smith 2013

- Family Endamoebidae Calkins 1926

- Order Entamoebida Cavalier-Smith 1993

- Order Pelobiontida Page 1976 emend. Cavalier Smith 1987

- Infraphylum Semiconosia Cavalier-Smith 2013

- Class Variosea Cavalier-Smith et al. 2004

- Order ?Flamellidae Cavalier-Smith 2016

- Order ?Holomastigida Lauterborn 1895 [Artodiscida Cavalier-Smith 2013]

- Order Phalansteriida Hibberd 1983

- Order Ramamoebida Cavalier-Smith 2016

- Order Profiliida Kang et al. 2017 [Protosteliida Olive & Stoianovitch 1966 em. Shadwick & Spiegel 2012]

- Order Fractovitellida Lahr et al. 2011 em. Kang et al. 2017

- Superclass Mycetozoa de Bary, 1859 ex Rostafinski, 1873

- Order Echinosteliopsida

- Class Dictyostelea Hawksworth et al. 1983

- Order Acytosteliales Baldauf, Sheikh & Thulin 2017

- Order Dictyosteliida Lister 1909 em. Olive 1970

- Class Ceratiomyxomycetes Hawksworth, Sutton & Ainsworth 1983

- Order Protosporangiida Shadwick & Spiegel 2012

- Order Ceratiomyxida Martin 1961 ex Farr & Alexopoulos

- Class Myxomycetes Link 1833 em. Haeckel 1866

- Subclass Lucisporomycetidae Leontyev et al. 2019

- Superorder Cribrarianae Leontyev 2015

- Order Cribrariales Macbr. 1922

- Superorder Trichianae Leontyev 2015

- Order Reticulariales Leontyev 2015

- Order Liceida Jahn 1928

- Order Trichiida Macbride 1922

- Subclass Columellomycetidae Leontyev et al. 2019

- Superorder Echinostelianae Leontyev 2015

- Order Echinosteliida Martin 1961

- Superorder Stemonitanae Leontyev 2015 [Fuscisporida Cavalier-Smith 2012]

- Order Clastodermatales Leontyev 2015

- Order Meridermatales Leontyev 2015

- Order Stemonitales Macbride 1922

- Order Physarales Macbride 1922

Fossil record[edit]

Vase-shaped microfossils (VSMs) discovered around the world show that amoebozoans have existed since the Neoproterozoic Era. The fossil species Melanocyrillium hexodiadema, Palaeoarcella athanata, and Hemisphaeriella ornata come from rocks 750 million years old. All three VSMs share a hemispherical shape, invaginated aperture, and regular indentations, that strongly resemble modern arcellinids, which are shell-bearing amoeboids. P. athanata in particular looks the same as the extant genus Arcella.[19][20]

List of amoebozoan protozoa pathogenic to humans[edit]

- Entamoeba histolytica

- Acanthamoeba

- Balamuthia mandrillaris

- Endolimax

Meiosis[edit]

The recently available Acanthamoeba genome sequence revealed several orthologs of genes employed in meiosis of sexual eukaryotes. These genes included Spo11, Mre11, Rad50, Rad51, Rad52, Mnd1, Dmc1, Msh and Mlh.[21] This finding suggests that Acanthamoeba is capable of some form of meiosis and may be able to undergo sexual reproduction.

In sexually reproducing eukaryotes, homologous recombination (HR) ordinarily occurs during meiosis. The meiosis-specific recombinase, Dmc1, is required for efficient meiotic HR, and Dmc1 is expressed in Entamoeba histolytica.[22] The purified Dmc1 from E. histolytica forms presynaptic filaments and catalyzes ATP-dependent homologous DNA pairing and DNA strand exchange over at least several thousand base pairs.[22] The DNA pairing and strand exchange reactions are enhanced by the eukaryotic meiosis-specific recombination accessory factor (heterodimer) Hop2-Mnd1.[22] These processes are central to meiotic recombination, suggesting that E. histolytica undergoes meiosis.[22]

Studies of Entamoeba invadens found that, during the conversion from the tetraploid uninucleate trophozoite to the tetranucleate cyst, homologous recombination is enhanced.[23] Expression of genes with functions related to the major steps of meiotic recombination also increased during encystations.[23] These findings in E. invadens, combined with evidence from studies of E. histolytica indicate the presence of meiosis in the Entamoeba. A comparative genetic analysis indicated that meiotic processes are present in all major amoebozoan lineages.[24]

Since Amoebozoa diverged early from the eukaryotic family tree, these results also suggest that meiosis was present early in eukaryotic evolution.

Human health[edit]

Amoebiasis, also known as amebiasis or entamoebiasis,[25][26] is an infection caused by any of the amoebozoans of the Entamoeba group. Symptoms are most common upon infection by Entamoeba histolytica. Amoebiasis can present with no, mild, or severe symptoms. Symptoms may include abdominal pain, mild diarrhoea, bloody diarrhea or severe colitis with tissue death and perforation. This last complication may cause peritonitis. People affected may develop anemia due to loss of blood.[27]

Invasion of the intestinal lining causes amoebic bloody diarrhea or amoebic colitis. If the parasite reaches the bloodstream it can spread through the body, most frequently ending up in the liver where it causes amoebic liver abscesses. Liver abscesses can occur without previous diarrhea. Cysts of Entamoeba can survive for up to a month in soil or for up to 45 minutes under fingernails. It is important to differentiate between amoebiasis and bacterial colitis. The preferred diagnostic method is through faecal examination under microscope, but requires a skilled microscopist and may not be reliable when excluding infection. This method however may not be able to separate between specific types. Increased white blood cell count is present in severe cases, but not in mild ones. The most accurate test is for antibodies in the blood, but it may remain positive following treatment.[27]

Prevention of amoebiasis is by separating food and water from faeces and by proper sanitation measures. There is no vaccine. There are two treatment options depending on the location of the infection. Amoebiasis in tissues is treated with either metronidazole, tinidazole, nitazoxanide, dehydroemetine or chloroquine, while luminal infection is treated with diloxanide furoate or iodoquinoline. For treatment to be effective against all stages of the amoeba may require a combination of medications. Infections without symptoms do not require treatment but infected individuals can spread the parasite to others and treatment can be considered. Treatment of other Entamoeba infections apart from E. histolytica is not needed.[27]

Amoebiasis is present all over the world.[28] About 480 million people are infected with what appears to be E. histolytica and these result in the death of between 40,000–110,000 people every year. Most infections are now ascribed to E. dispar. E. dispar is more common in certain areas and symptomatic cases may be fewer than previously reported. The first case of amoebiasis was documented in 1875 and in 1891 the disease was described in detail, resulting in the terms amoebic dysentery and amoebic liver abscess. Further evidence from the Philippines in 1913 found that upon ingesting cysts of E. histolytica volunteers developed the disease. It has been known since 1897 that at least one non-disease-causing species of Entamoeba existed (Entamoeba coli), but it was first formally recognized by the WHO in 1997 that E. histolytica was two species, despite this having first been proposed in 1925. In addition to the now-recognized E. dispar evidence shows there are at least two other species of Entamoeba that look the same in humans - E. moshkovskii and Entamoeba bangladeshi. The reason these species haven't been differentiated until recently is because of the reliance on appearance.[27]

Gallery[edit]

- Amoeba proteus (Lobosa: Tubulinea)

- Arcella sp. test (Lobosa: Tubulinea)

- Acanthamoeba sp. (Lobosa: Discosea)

- Thecamoeba sp. (Lobosa: Discosea)

- Entamoeba histolytica trophozoite (Conosa: Archamoebae)

- Pelomyxa palustris (Conosa: Archamoebae)

- Stemonitis sp. (Conosa: Myxogastria)

- Dictyostelium discoideum (Conosa: Dictyostelia)

References

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Rotifer

From Wikipedia, the free encyclopedia

Rotifera

Temporal range: Eocene–Recent

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Mikrofoto.de-Raedertier-14.jpg

Rotifera

Pulchritia dorsicornuta - ZooKeys-342-001-g001.jpg

Pulchritia dorsicornuta

Scientific classification

e

Kingdom:

Animalia

Clade:

Gnathifera

Phylum:

Rotifera

Cuvier, 1798

Classes and other subgroups

- Eurotatoria

- Bdelloidea

- Monogononta

- Pararotatoria

- Seisonidae

- and See text.

The rotifers (from Latin rota "wheel" and -fer "bearing"), commonly called wheel animals or wheel animalcules,[1] make up a phylum (Rotifera) of microscopic and near-microscopic pseudocoelomate animals.

They were first described by Rev. John Harris in 1696, and other forms were described by Antonie van Leeuwenhoek in 1703.[2] Most rotifers are around 0.1–0.5 mm long (although their size can range from 50 μm to over 2 mm),[1] and are common in freshwater environments throughout the world with a few saltwater species.

Some rotifers are free swimming and truly planktonic, others move by inchworming along a substrate, and some are sessile, living inside tubes or gelatinous holdfasts that are attached to a substrate. About 25 species are colonial (e.g., Sinantherina semibullata), either sessile or planktonic. Rotifers are an important part of the freshwater zooplankton, being a major foodsource and with many species also contributing to the decomposition of soil organic matter.[3] Most species of the rotifers are cosmopolitan, but there are also some endemic species, like Cephalodella vittata to Lake Baikal.[4] Recent barcoding evidence, however, suggests that some 'cosmopolitan' species, such as Brachionus plicatilis, B. calyciflorus, Lecane bulla, among others, are actually species complexes.[5][6]

In some recent treatments, rotifers are placed with acanthocephalans in a larger clade called Syndermata.

Contents

- 1Taxonomy and naming

- 1.1Etymology

- 2Anatomy

- 2.1Digestive system

- 2.2Nervous system

- 2.3Biology

- 3Feeding

- 4Reproduction and life cycle

- 4.1Loss of sexual reproduction system

- 4.2Resting eggs

- 4.3Anhydrobiosis

- 5Predators

- 6Genome size

- 7Gallery

- 8References

- 9External links

Taxonomy and naming[edit]

See also: List of bilaterial animal orders

Individual rotifers

A bdelloid rotifer

Ptygura pilula

Brachionus quadridentatus

Rev. John Harris first described the rotifers (in particular a bdelloid rotifer) in 1696 as "an animal like a large maggot which could contract itself into a spherical figure and then stretch itself out again; the end of its tail appeared with a forceps like that of an earwig".[2] In 1702, Antonie van Leeuwenhoek gave a detailed description of Rotifer vulgaris and subsequently described Melicerta ringens and other species.[7] He was also the first to publish observations of the revivification of certain species after drying. Other forms were described by other observers, but it wasn't until the publication of Christian Gottfried Ehrenberg's Die Infusionsthierchen als vollkommene Organismen in 1838 that the rotifers were recognized as being multicellular animals.[7]

About 2200 species of rotifers have been described. Their taxonomy is currently in a state of flux. One treatment places them in the phylum Rotifera, with three classes: Seisonidea, Bdelloidea and Monogononta.[8] The largest group is the Monogononta, with about 1500 species, followed by the Bdelloidea, with about 350 species. There are only two known genera with three species of Seisonidea.[9]

The Acanthocephala, previously considered to be a separate phylum, have been demonstrated to be modified rotifers. The exact relationship to other members of the phylum has not yet been resolved.[10] One possibility is that the Acanthocephala are closer to the Bdelloidea and Monogononta than to the Seisonidea; the corresponding names and relationships are shown in the cladogram below.

Syndermata

Seisonida

Eurotatoria

Bdelloidea

Monogononta

Acanthocephala

The Rotifera, strictly speaking, are confined to the Bdelloidea and the Monogononta. Rotifera, Acanthocephala and Seisonida make up a clade called Syndermata.[11]

Etymology[edit]

The word rotifer is derived from a Neo-Latin word meaning "wheel-bearer",[12] due to the corona around the mouth that in concerted sequential motion resembles a wheel (though the organ does not actually rotate).

Anatomy[edit]

Rotifer colonies

Colonial rotifers, tentatively identified as Conochilus from Lake Pontchartrain, Louisiana: the colony is somewhat less than 1 mm in diameter, but visible to the naked eye.

A colony of Sinantheria socialis on an Elodea densa leaf from North German Lake. Note heart-shaped corona of individuals.

Rotifers have bilateral symmetry and a variety of different shapes. The body of a rotifer is divided into a head, trunk, and foot, and is typically somewhat cylindrical. There is a well-developed cuticle, which may be thick and rigid, giving the animal a box-like shape, or flexible, giving the animal a worm-like shape; such rotifers are respectively called loricate and illoricate. Rigid cuticles are often composed of multiple plates, and may bear spines, ridges, or other ornamentation. Their cuticle is nonchitinous and is formed from sclerotized proteins.

The most distinctive feature of rotifers is the presence of a ciliated structure, called the corona, on the head. In the more primitive species, this forms a simple ring of cilia around the mouth from which an additional band of cilia stretches over the back of the head. In the great majority of rotifers, however, this has evolved into a more complex structure.

Modifications to the basic plan of the corona include alteration of the cilia into bristles or large tufts, and either expansion or loss of the ciliated band around the head. In genera such as Collotheca, the corona is modified to form a funnel surrounding the mouth. In many species, such as those in the genus Testudinella, the cilia around the mouth have disappeared, leaving just two small circular bands on the head. In the bdelloids, this plan is further modified, with the upper band splitting into two rotating wheels, raised up on a pedestal projecting from the upper surface of the head.[13]

The trunk forms the major part of the body, and encloses most of the internal organs. The foot projects from the rear of the trunk, and is usually much narrower, giving the appearance of a tail. The cuticle over the foot often forms rings, making it appear segmented, although the internal structure is uniform. Many rotifers can retract the foot partially or wholly into the trunk. The foot ends in from one to four toes, which, in sessile and crawling species, contain adhesive glands to attach the animal to the substratum. In many free-swimming species, the foot as a whole is reduced in size, and may even be absent.[13]

Digestive system[edit]

Scanning electron micrographs showing morphological variation of bdelloid rotifers and their jaws.

The coronal cilia create a current that sweeps food into the mouth. The mouth opens into a characteristic chewing pharynx (called the mastax), sometimes via a ciliated tube, and sometimes directly. The pharynx has a powerful muscular wall and contains tiny, calcified, jaw-like structures called trophi, which are the only fossilizable parts of a rotifer. The shape of the trophi varies between different species, depending partly on the nature of their diet. In suspension feeders, the trophi are covered in grinding ridges, while in more actively carnivorous species, they may be shaped like forceps to help bite into prey. In some ectoparasitic rotifers, the mastax is adapted to grip onto the host, although, in others, the foot performs this function instead.[13]

Behind the mastax lies an oesophagus, which opens into a stomach where most of the digestion and absorption occurs. The stomach opens into a short intestine that terminates in a cloaca on the posterior dorsal surface of the animal. Up to seven salivary glands are present in some species, emptying to the mouth in front of the oesophagus, while the stomach is associated with two gastric glands that produce digestive enzymes.[13]

A pair of protonephridia open into a bladder that drains into the cloaca. These organs expel water from the body, helping to maintain osmotic balance.[13]

Nervous system[edit]

Rotifers have a small brain, located just above the mastax, from which a number of nerves extend throughout the body. The number of nerves varies among species, although the nervous system usually has a simple layout. Close to the brain lies a retrocerebral organ, consisting of two glands either side of a medial sac. The sac drains into a duct that divides into two before opening through pores on the uppermost part of the head. The function of the retrocerebral organ is unclear.[13][14]

The nervous system comprises about 25% of the roughly 1,000 cells in a rotifer.[14]

Rotifers typically possess one or two pairs of short antennae and up to five eyes. The eyes are simple in structure, sometimes with just a single photoreceptor cell. In addition, the bristles of the corona are sensitive to touch, and there are also a pair of tiny sensory pits lined by cilia in the head region.[13]

Biology[edit]

The coronal cilia pull the animal, when unattached, through the water.

Like many other microscopic animals, adult rotifers frequently exhibit eutely—they have a fixed number of cells within a species, usually on the order of 1,000.

Bdelloid rotifer genomes contain two or more divergent copies of each gene, suggesting a long-term asexual evolutionary history.[15] For example, four copies of hsp82 are found. Each is different and found on a different chromosome excluding the possibility of homozygous sexual reproduction.

Feeding[edit]

Video of rotifer feeding, probably of the genus Cephalodella

Video of a bdelloid rotifer feeding

Rotifers eat particulate organic detritus, dead bacteria, algae, and protozoans. They eat particles up to 10 micrometres in size. Like crustaceans, rotifers contribute to nutrient recycling. For this reason, they are used in fish tanks to help clean the water, to prevent clouds of waste matter. Rotifers affect the species composition of algae in ecosystems through their choice in grazing. Rotifers may be in competition with cladocera and copepods for planktonic food sources.

Reproduction and life cycle[edit]

Rotifers are dioecious and reproduce sexually or parthenogenetically. They are sexually dimorphic, with the females always being larger than the males. In some species, this is relatively mild, but in others the female may be up to ten times the size of the male. In parthenogenetic species, males may be present only at certain times of the year, or absent altogether.[13]

The female reproductive system consists of one or two ovaries, each with a vitellarium gland that supplies the eggs with yolk. Together, each ovary and vitellarium form a single syncitial structure in the anterior part of the animal, opening through an oviduct into the cloaca.[13]

Males do not usually have a functional digestive system, and are therefore short-lived, often being sexually fertile at birth. They have a single testicle and sperm duct, associated with a pair of glandular structures referred to as prostates (unrelated to the vertebrate prostate). The sperm duct opens into a gonopore at the posterior end of the animal, which is usually modified to form a penis. The gonopore is homologous to the cloaca of females, but in most species has no connection to the vestigial digestive system, which lacks an anus.[13]

The phylum Rotifera encloses three classes that reproduce by three different mechanisms: Seisonidea only reproduce sexually; Bdelloidea reproduce exclusively by asexual parthenogenesis; Monogononta reproduce alternating these two mechanisms ("cyclical parthenogenesis" or "heterogony").[16] Parthenogenesis (amictic phase) dominates the monogonont life cycle, promoting fast population growth and colonization. In this phase males are absent and amictic females produce diploid eggs by mitosis which develop parthenogenetically into females that are clones of their mothers.[16] Some amictic females can generate mictic females that will produce haploid eggs by meiosis. Mixis (meiosis) is induced by different types of stimulus depending on species. Haploid eggs develop into haploid dwarf males if they are not fertilized and into diploid "resting eggs" (or "diapausing eggs") if they are fertilized by males.

Fertilization is internal. The male either inserts his penis into the female's cloaca or uses it to penetrate her skin, injecting the sperm into the body cavity. The egg secretes a shell, and is attached either to the substratum, nearby plants, or the female's own body. A few species, such as members of the Rotaria, are ovoviviparous, retaining the eggs inside their body until they hatch.[13]

Most species hatch as miniature versions of the adult. Sessile species, however, are born as free-swimming larvae, which closely resemble the adults of related free-swimming species. Females grow rapidly, reaching their adult size within a few days, while males typically do not grow in size at all.[13]

The life span of monogonont females varies from two days to about three weeks.

Loss of sexual reproduction system[edit]

'Ancient asexuals': Bdelloid rotifers are assumed to have reproduced without sex for many millions of years. Males are absent within the species, and females reproduce only by parthenogenesis.

Recent transitions: Loss of sexual reproduction can be inherited in a simple Mendelian fashion in the monogonont rotifer Brachionus calyciflorus: This species can normally switch between sexual and asexual reproduction (cyclical parthenogenesis), but occasionally gives rise to purely asexual lineages (obligate parthenogens). These lineages are unable to reproduce sexually due to being homozygous for a recessive allele.[17]

Resting eggs[edit]

Resting eggs enclose an embryo encysted in a three layered shell that protects it from external stressors.[18][19] They are able to remain dormant for several decades and can resist adverse periods (e.g., pond desiccation or presence of antagonists).[20][21] When favourable conditions return and after an obligatory period of diapause which varies among species, resting eggs hatch releasing diploid amictic females that enter into the asexual phase of the life cycle.[16][22]

Anhydrobiosis[edit]

Bdelloid rotifer females cannot produce resting eggs, but many can survive prolonged periods of adverse conditions after desiccation. This facility is termed anhydrobiosis, and organisms with these capabilities are termed anhydrobionts. Under drought conditions, bdelloid rotifers contract into an inert form and lose almost all body water; when rehydrated they resume activity within a few hours. Bdelloids can survive the dry state for long periods, with the longest well-documented dormancy being nine years. While in other anhydrobionts, such as the brine shrimp, this desiccation tolerance is thought to be linked to the production of trehalose, a non-reducing disaccharide (sugar), bdelloids apparently cannot synthesise trehalose. In bdelloids, a major cause of the resistance to desiccation, as well as resistance to ionizing radiation, is a highly efficient mechanism for repairing the DNA double-strand breaks induced by these agents.[23] This repair mechanism likely involves mitotic recombination between homologous DNA regions.[23]

Predators[edit]

Rotifers fall prey to many animals, such as copepods, fish (e.g. herring, salmon), bryozoa, comb jellies, jellyfish, starfish, and tardigrade.[24]

Genome size[edit]

The genome size of a bdelloid rotifer, Adineta vaga, was reported to be around 244 Mb.[25] The genomes of Monogononts seem to be significantly smaller than those of Bdelloids. In Monogononta the nuclear DNA content (2C) in eight different species of four different genera ranged almost fourfold, from 0.12 to 0.46 pg.[26] Haploid "1C" genome sizes in Brachionus species range at least from 0.056 to 0.416 pg.[27]

Gallery[edit]

- Pair of Lepadella Rotifers, from pond water in Northeast US.

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Ciliate

From Wikipedia, the free encyclopedia

This article is about the protozoan phylum Ciliophora. For the type of ciliate cells in general, see Ciliate cells. For the type of leaf margin, see Leaf § Margins (edge).

Ciliate

Temporal range: Ediacaran–Recent

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Ciliate collage.jpg

Some examples of ciliate diversity. Clockwise from top left: Lacrymaria, Coleps, Stentor, Dileptus, Paramecium

Scientific classification

e

(unranked):

Diaphoretickes

Clade:

TSAR

Clade:

SAR

Infrakingdom:

Alveolata

Phylum:

Ciliophora

Doflein, 1901 emend.

Subphyla and Classes[1]

- Postciliodesmatophora

- Heterotrichea

- Karyorelictea

- Intramacronucleata

- Armophorea

- Litostomatea

- Colpodea

- Nassophorea

- Phyllopharyngea

- Prostomatea

- Plagiopylea

- Oligohymenophorea

- Protocruziea

- Spirotrichea

- Cariacotrichea

- Muranotrichea

- Parablepharismea

See text for subclasses.

Synonyms

- Ciliata Perty, 1852

The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different undulating pattern than flagella. Cilia occur in all members of the group (although the peculiar Suctoria only have them for part of their life-cycle) and are variously used in swimming, crawling, attachment, feeding, and sensation.

Ciliates are an important group of protists, common almost anywhere there is water — in lakes, ponds, oceans, rivers, and soils. About 4,500 unique free-living species have been described, and the potential number of extant species is estimated at 27,000–40,000.[2] Included in this number are many ectosymbiotic and endosymbiotic species, as well as some obligate and opportunistic parasites. Ciliate species range in size from as little as 10 µm in some colpodeans to as much as 4 mm in length in some geleiids, and include some of the most morphologically complex protozoans.[3][4]

In most systems of taxonomy, "Ciliophora" is ranked as a phylum[5] under any of several kingdoms, including Chromista[6], Protista[7] or Protozoa.[8] In some older systems of classification, such as the influential taxonomic works of Alfred Kahl, ciliated protozoa are placed within the class "Ciliata"[9][10] (a term which can also refer to a genus of fish). In the taxonomic scheme endorsed by the International Society of Protistologists, which eliminates formal rank designations such as "phylum" and "class", "Ciliophora" is an unranked taxon within Alveolata.[11][12]

Contents

- 1Cell structure

- 1.1Nuclei

- 1.2Cytoplasm

- 1.3Specialized structures in ciliates

- 2Feeding

- 3Reproduction and sexual phenomena

- 3.1Reproduction

- 3.2Conjugation

- 4DNA rearrangements (gene scrambling)

- 5Fossil record

- 6Classification

- 6.1Subphylum Postciliodesmatophora

- 6.2Subphylum Intramacronucleata

- 6.3Other

- 7Pathogenicity

- 8References

- 9Further reading

Cell structure[edit]

Diagram of the ciliate Paramecium

Nuclei[edit]

Unlike most other eukaryotes, ciliates have two different sorts of nuclei: a tiny, diploid micronucleus (the "generative nucleus," which carries the germline of the cell), and a large, polyploid macronucleus (the "vegetative nucleus," which takes care of general cell regulation, expressing the phenotype of the organism). The latter is generated from the micronucleus by amplification of the genome and heavy editing. The micronucleus passes its genetic material to offspring, but does not express its genes. The macronucleus provides the small nuclear RNA for vegetative growth.

Division of the macronucleus occurs by amitosis, and the segregation of the chromosomes occurs by a process whose mechanism is unknown. This process is not perfect, and after about 200 generations the cell shows signs of aging. Periodically the macronuclei must be regenerated from the micronuclei. In most, this occurs during conjugation. Here two cells line up, the micronuclei undergo meiosis, some of the haploid daughters are exchanged and then fuse to form new micronuclei and macronuclei.

Cytoplasm[edit]

Food vacuoles are formed through phagocytosis and typically follow a particular path through the cell as their contents are digested and broken down by lysosomes so the substances the vacuole contains are then small enough to diffuse through the membrane of the food vacuole into the cell. Anything left in the food vacuole by the time it reaches the cytoproct (anal pore) is discharged by exocytosis. Most ciliates also have one or more prominent contractile vacuoles, which collect water and expel it from the cell to maintain osmotic pressure, or in some function to maintain ionic balance. In some genera, such as Paramecium, these have a distinctive star shape, with each point being a collecting tube.

Specialized structures in ciliates[edit]

Cilia are arranged in rows called kineties. In some forms there are also body polykinetids, for instance, among the spirotrichs where they generally form bristles called cirri. More often body cilia are arranged in mono- and dikinetids, which respectively include one and two kinetosomes (basal bodies), each of which may support a cilium. These are arranged into rows called kineties, which run from the anterior to posterior of the cell. The body and oral kinetids make up the infraciliature, an organization unique to the ciliates and important in their classification, and include various fibrils and microtubules involved in coordinating the cilia.

The infraciliature is one of the main components of the cell cortex. Others are the alveoli, small vesicles under the cell membrane that are packed against it to form a pellicle maintaining the cell's shape, which varies from flexible and contractile to rigid. Numerous mitochondria and extrusomes are also generally present. The presence of alveoli, the structure of the cilia, the form of mitosis and various other details indicate a close relationship between the ciliates, Apicomplexa, and dinoflagellates. These superficially dissimilar groups make up the alveolates.

Feeding[edit]

Most ciliates are heterotrophs, feeding on smaller organisms, such as bacteria and algae, and detritus swept into the oral groove (mouth) by modified oral cilia. This usually includes a series of membranelles to the left of the mouth and a paroral membrane to its right, both of which arise from polykinetids, groups of many cilia together with associated structures. The food is moved by the cilia through the mouth pore into the gullet, which forms food vacuoles.

Feeding techniques vary considerably, however. Some ciliates are mouthless and feed by absorption (osmotrophy), while others are predatory and feed on other protozoa and in particular on other ciliates. Some ciliates parasitize animals, although only one species, Balantidium coli, is known to cause disease in humans.[13]

Reproduction and sexual phenomena[edit]

Most ciliates divide transversally, but other kinds of binary fission occur in some species

Reproduction[edit]

Ciliates reproduce asexually, by various kinds of fission.[14] During fission, the micronucleus undergoes mitosis and the macronucleus elongates and undergoes amitosis (except among the Karyorelictean ciliates, whose macronuclei do not divide). The cell then divides in two, and each new cell obtains a copy of the micronucleus and the macronucleus.

Ciliate undergoing the last processes of binary fission

Typically, the cell is divided transversally, with the anterior half of the ciliate (the proter) forming one new organism, and the posterior half (the opisthe) forming another. However, other types of fission occur in some ciliate groups. These include budding (the emergence of small ciliated offspring, or "swarmers", from the body of a mature parent); strobilation (multiple divisions along the cell body, producing a chain of new organisms); and palintomy (multiple fissions, usually within a cyst).[15]

Fission may occur spontaneously, as part of the vegetative cell cycle. Alternatively, it may proceed as a result of self-fertilization (autogamy),[16] or it may follow conjugation, a sexual phenomenon in which ciliates of compatible mating types exchange genetic material. While conjugation is sometimes described as a form of reproduction, it is not directly connected with reproductive processes, and does not directly result in an increase in the number of individual ciliates or their progeny.[17]

Conjugation[edit]

Overview

Ciliate conjugation is a sexual phenomenon that results in genetic recombination and nuclear reorganization within the cell.[17][15] During conjugation, two ciliates of a compatible mating type form a bridge between their cytoplasms. The micronuclei undergo meiosis, the macronuclei disappear, and haploid micronuclei are exchanged over the bridge. In some ciliates (peritrichs, chonotrichs and some suctorians), conjugating cells become permanently fused, and one conjugant is absorbed by the other.[13][18] In most ciliate groups, however, the cells separate after conjugation, and both form new macronuclei from their micronuclei.[19] Conjugation and autogamy are always followed by fission.[15]

In many ciliates, such as Paramecium, conjugating partners (gamonts) are similar or indistinguishable in size and shape. This is referred to as "isogamontic" conjugation. In some groups, partners are different in size and shape. This is referred to as "anisogamontic" conjugation. In sessile peritrichs, for instance, one sexual partner (the microconjugant) is small and mobile, while the other (macroconjugant) is large and sessile.[17]

Stages of conjugation

Stages of conjugation in Paramecium caudatum

In Paramecium caudatum, the stages of conjugation are as follows (see diagram at right):

1. Compatible mating strains meet and partly fuse

2. The micronuclei undergo meiosis, producing four haploid micronuclei per cell.

3. Three of these micronuclei disintegrate. The fourth undergoes mitosis.

4. The two cells exchange a micronucleus.

5. The cells then separate.

6. The micronuclei in each cell fuse, forming a diploid micronucleus.

7. Mitosis occurs three times, giving rise to eight micronuclei.

8. Four of the new micronuclei transform into macronuclei, and the old macronucleus disintegrates.

9. Binary fission occurs twice, yielding four identical daughter cells.

DNA rearrangements (gene scrambling)[edit]

Ciliates contain two types of nuclei: somatic "macronucleus" and the germline "micronucleus". Only the DNA in the micronucleus is passed on during sexual reproduction (conjugation). On the other hand, only the DNA in the macronucleus is actively expressed and results in the phenotype of the organism. Macronuclear DNA is derived from micronuclear DNA by amazingly extensive DNA rearrangement and amplification.

Development of the Oxytricha macronuclear genome from micronuclear genome

The macronucleus begins as a copy of the micronucleus. The micronuclear chromosomes are fragmented into many smaller pieces and amplified to give many copies. The resulting macronuclear chromosomes often contain only a single gene. In Tetrahymena, the micronucleus has 10 chromosomes (five per haploid genome), while the macronucleus has over 20,000 chromosomes.[20]

In addition, the micronuclear genes are interrupted by numerous "internal eliminated sequences" (IESs). During development of the macronucleus, IESs are deleted and the remaining gene segments, macronuclear destined sequences (MDSs), are spliced together to give the operational gene. Tetrahymena has about 6,000 IESs and about 15% of micronuclear DNA is eliminated during this process. The process is guided by small RNAs and epigenetic chromatin marks.[20]

In spirotrich ciliates (such as Oxytricha), the process is even more complex due to "gene scrambling": the MDSs in the micronucleus are often in different order and orientation from that in the macronuclear gene, and so in addition to deletion, DNA inversion and translocation are required for "unscrambling". This process is guided by long RNAs derived from the parental macronucleus. More than 95% of micronuclear DNA is eliminated during spirotrich macronuclear development.[20]

Fossil record[edit]

Until recently, the oldest ciliate fossils known were tintinnids from the Ordovician period. In 2007, Li et al. published a description of fossil ciliates from the Doushantuo Formation, about 580 million years ago, in the Ediacaran period. These included two types of tintinnids and a possible ancestral suctorian.[21] A fossil Vorticella has been discovered inside a leech cocoon from the Triassic period, about 200 million years ago.[22]

Classification

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