| Gregarinasina | |
|---|---|
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| A live specimen of a septate (orcephaline) gregarine showing the distinctive "head"-like section of the trophozoite containing the epimerite at its anterior end. Septate gregarines are intestinalparasites ofarthropods. | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Sar |
| Clade: | Alveolata |
| Phylum: | Apicomplexa |
| Class: | Conoidasida |
| Subclass: | Gregarinasina |
| Orders | |
| Synonyms | |
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Thegregarines are a group ofApicomplexan alveolates, classified as theGregarinasina[1] orGregarinia. The large (roughly half a millimeter) parasites inhabit the intestines of many invertebrates. They are not found in any vertebrates. Gregarines are closely related to bothToxoplasma andPlasmodium, which causetoxoplasmosis andmalaria, respectively. Both protists useprotein complexes similar to those that are formed by the gregarines forgliding motility and for invading target cells.[2][3] This makes the gregarines excellent models for studying gliding motility, with the goal of developing treatment options for both toxoplasmosis and malaria.Thousands of different species of gregarine are expected to be found ininsects, and 99% of these gregarine species still need to be described. Each insect species can be the host of multiple gregarine species.[4][5] One of the most-studied gregarines isGregarina garnhami.In general, gregarines are regarded as a very successful group of parasites, as their hosts are distributed over the entire planet.[6]
Gregarines occur in both aquatic and terrestrial environments. Although they are usually transmitted by theorofaecal route, some are transmitted with thehost'sgametes duringcopulation,e.g.,Monocystis.
In all species, four or moresporozoites (depending on the species), equipped with anapical complex, escape from theoocysts in a process called excystation. They find their way to the appropriate body cavity, and penetrate host cells in their immediate environment. The sporozoites begin to feed within the host cell and develop into largertrophozoites. In some species, the sporozoites and trophozoites are capable of asexual replication – a process calledschizogony ormerogony.
In all species, two mature trophozoites eventually pair up in a process known assyzygy and develop intogamonts. During syzygy, gamont orientation differs between species (side-to-side, head-to-tail). Agametocyst wall forms around each gamont pair, which then begins to divide into hundreds ofgametes.Zygotes are produced by the fusion of two gametes, and these, in turn, become surrounded by an oocyst wall. Within the oocyst,meiosis occurs, yielding the sporozoites. Hundreds of oocysts accumulate within each gametocyst; these are released via a host'sfaeces or via host death and decay.
Gregarines have thus far been reported to infect over 3000 invertebrate species.[7]
The gregarines were recognised as ataxon byGrasse in 1953.[8] The three orders into which they are currently divided were created by Levineet al. in 1980.
Currently, about 250 genera and 1650 species are known in this taxon. They are divided into three orders based on habitat, host range, and trophozoite morphology.[9]
Most species havemonoxenous lifecycles involving a single invertebrate host. In the lifecycle, the extracellular feeding stage is known as the trophozoite.
Archigregarines are found only in marine habitats. They possess intestinal trophozoites similar in morphology to the infective sporozoites. Phylogenetic analysis suggests this group is paraphyletic and will need division. Generally, four zoites are in each spore in this group.
Eugregarines are found in marine, freshwater, and terrestrial habitats. These species possess large trophozoites that are significantly different in morphology and behavior from the sporozoites. This taxon contains most of the known gregarine species. The intestinaleugregarines are separated into septate – suborderSeptatorina – andaseptate – suborderAseptatorina – depending on whether the trophozoite is superficially divided by a transverse septum. The aseptate species are mostly marine gregarines.
Urosporidians are aseptate eugregarines that infect the coelomic spaces of marine hosts. Unusually, they tend to lack attachment structures and form gamont pairs that pulsate freely within thecoelomic fluid.
Monocystids are aseptate eugregarines that infect the reproductive vesicles of terrestrialannelids. These latter species tend to branch closely withneogregarines and may need to be reclassified. Generally, eight zoites are in each spore in this group.
Neogregarines are found only in terrestrial hosts. These species have reduced trophozoites and tend to infect tissues other than the intestine. Usually, eight zoites are in each spore in this group.
The eugregarines and neogregarines differ in a number of respects. The neogregarines are in general more pathogenic to their hosts. The eugregarines multiply by sporogony and gametogony, while the neogregarines have an additional schizogenic stage – merogony – within their hosts. Merogony may be intracellular or extracellular depending on the species.
DNA studies suggest thearchigregarines are ancestral to the others.[10]
Cavalier-Smith has proposed a significant revision of this taxon assuming thepolyphyly ofeugregarines.[11] He has separated gregarines into three classes. The first of them –Gregarinomorphea – comprisesOrthogregarinia,Cryptosporidiidae and, additionally,Rhytidocystidae previously considered as divergentcoccidians[9] orApicomplexaincertae sedis.[12] The Orthogregarinia with two new ordersArthrogregarida andVermigregarida was created for the gregarines most closely related toCryptosporidium. The second class –Paragregarea – was created for the archigregarines,Stenophorida and a new order –Velocida which itself was created forUrosporoideasuperfam. n. andVeloxidium. The third class was created –Squirmida – forFilipodium andPlatyproteum. Thus, theeugregarines proved to be split and distributed among these three classes together with some otherapicomplexans.
This point of view was challenged in 2017 by Simdyanov and co-authors, who performed the global integrated analysis of available morphological and molecular phylogenetic data and concluded thateugregarines are rather amonophyletic taxon.[13]
Several genera of gregarines are currently not classified:Acuta,Cephalolobus,Gregarina,Levinea,Menospora,Nematocystis,Nematopsis,Steinina, andTrichorhynchus.
The parasites are relatively large, spindle-shaped cells, compared to other apicomplexans and eukaryotes in general (some species are> 850µm in length). Most gregarines have longitudinal epicytic folds (bundles ofmicrotubules beneath the cell surface with nematode like bending behaviour): crenulations are instead found in the urosporidians.
The gregarines are able to move and change direction along a surface through gliding motility without the use ofcilia,flagella, orlamellipodia.[14] This is accomplished through the use of anactin andmyosin complex.[15] The complexes require an actincytoskeleton to perform their gliding motions.[16] In the proposed 'capping' model, an uncharacterized protein complex moves rearward, moving the parasites forward.[17]
The gregarines are among the oldest known parasites, having been described by the physicianFrancesco Redi in 1684.[18]
The first formal description was made byDufour in 1828. He created the genusGregarina, named for their gregarious nature[notes 1],and describedGregarina ovata fromFolficula aricularia. He considered them to be parasitic worms.Kölliker recognised them as protozoa in 1848[19].
