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The gregarines are a group of alveolates, classified as the Gregarinasina or Gregarinia. 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 both Toxoplasma and Plasmodium, which cause toxoplasmosis and malaria, respectively. Both protists use similar to those that are formed by the gregarines for gliding motility and for invading target cells. 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 in , and 99% of these gregarine species still need to be described. Each insect species can be the host of multiple gregarine species. One of the most-studied gregarines is Gregarina garnhami. In general, gregarines are regarded as a very successful group of parasites, as their hosts are distributed over the entire planet.
In all species, four or more (depending on the species), equipped with an apical complex, escape from the 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 larger . In some species, the sporozoites and trophozoites are capable of asexual replication – a process called schizogony or merogony.
In all species, two mature trophozoites eventually pair up in a process known as Meiosis and develop into gametocyte. During syzygy, gamont orientation differs between species (side-to-side, head-to-tail). A gametocyst wall forms around each gamont pair, which then begins to divide into hundreds of . 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's faeces or via host death and decay.
Gregarines have thus far been reported to infect over 3000 invertebrate species.Alarcón M E., Huang C-G, Tsai Y-S, Chen W-J, Kumar A (2011) Life cycle and morphology of Steinina ctenocephali (Ross 1909) comb. nov. (Eugregarinorida: Actinocephalidae), a gregarine of Ctenocephalides felis (Siphonaptera: Pulicidae) in Taiwan. Zoological Studies 50(6): 763-772
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. (2026). 9781891276224, Society of Protozoologists. ISBN 9781891276224
Most species have monoxenous lifecycles involving a single invertebrate host. In the lifecycle, the extracellular feeding stage is known as the trophozoite.
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 intestinal Eugregarinorida are separated into septate – suborder Septatorina – and aseptate – suborder Aseptatorina – 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 the coelomic fluid.
Monocystids are aseptate eugregarines that infect the reproductive vesicles of terrestrial . These latter species tend to branch closely with neogregarines 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 the archigregarines are ancestral to the others.
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 that Eugregarinorida are rather a monophyletic taxon.
Several genera of gregarines are currently not classified: Acuta, Cephalolobus, Gregarina, Levinea, Menospora, Nematocystis, Nematopsis, Steinina, and Trichorhynchus.
The parasites are relatively large, spindle-shaped cells, compared to other apicomplexans and eukaryotes in general (some species are in length). Most gregarines have longitudinal epicytic folds (bundles of beneath the cell surface with nematode like bending behaviour): crenulations are instead found in the urosporidians.
The first formal description was made by Dufour in 1828. He created the genus Gregarina, named for their gregarious nature, and described Gregarina ovata from Folficula aricularia. He considered them to be parasitic worms. Kölliker recognised them as protozoa in 1848.
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