|A live specimen of a septate (or cephaline) gregarine showing the distinctive "head"-like section of the trophozoite containing the epimerite at its anterior end. Septate gregarines are intestinal parasites of arthropods.|
The gregarines are a group of Apicomplexan protozoa, classified as the Gregarinasina or Gregarinia. The large (roughly half a millimeter) parasites inhabit the intestines of a large number of invertebrates. They are not found in any vertebrates. However, Gregarinasina is closely related to both Toxoplasma and Plasmodium, which cause toxoplasmosis and malaria, respectively. Both protists use protein complexes similar to those that are formed by the gregarines for gliding motility and invading target cells. This makes them an excellent model for studying gliding motility with the goal of developing toxoplasmosis and malaria treatment options.
Gregarines occur in both aquatic and terrestrial environments. Although they are usually transmitted by the orofaecal route some are transmitted with the host's gametes during copulation (Monocystis).
In all species four or more sporozoites (the precise number depends on the species) equipped with an apical complex escape from the oocysts, a process called excystation, ﬁnd their way to the appropriate body cavity and penetrate host cells in their immediate environment. The sporozoites emerge within the host cell, begin to feed and develop into larger trophozoites. In some species, the sporozoites and trophozoites are capable of asexual replication — a process called schizogony or merogony. Most species however appear to lack schizogony in their lifecycles.
In all species two mature trophozoites eventually pair up in a process known as syzygy and develop into gamonts. During syzygy gamont orientation differs between species (side to side, head to tail). A gametocyst wall forms around each pair of gamonts which then begin to divide into hundreds of gametes. 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 and these are released via host's faeces or via host death and decay.
There are currently approximately 250 genera and 1650 species known in this taxon. They are divided into three orders based on habitat, host range and trophozoite morphology.
Archigregarines are found only in marine habitats. They possess intestinal trophozoites that are similar in morphology to the infective sporozoites. Phylogenetic analysis suggests that this group is paraphyletic and will need division.
There are generally four zoites 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 intestinal eugregarines are separated into septate — suborder Septatina — and aseptate — suborder Aseptatina — 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.
There are generally eight zoites 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.
There are generally eight zoites 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.
- Meiosis occurs in all species
- Monoxenous — only one host in life cycle — almost all species
- Mitochondria have tubular cristae and are often distributed near the cell periphery
- Apical complex occurs in the sporozoite stage; lost in the trophozoite stage in eugregarines and neogregarines
- Trophozoites have a large and conspicuous nucleus and nucleolus
- Inhabit extracellular body cavities of invertebrates such as the intestines, coeloms and reproductive vesicles
- Attachment to host via a mucron (aseptate gregarines) or an epimerite (septate gregarines); some gregarines (urosporidians) float freely within extracellular body cavities (coelom)
The parasites are relatively large spindle-shaped cells, compared to other apicomplexans and eukaryotes in general (some species are > 850 µm in length). Most gregarines have longitudinal epicytic folds (bundles of microtubules 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 of cilia, flagella, or lamellipodia. This is accomplished through the use of an actin and myosin complex. The complexes require an actin cytoskeleton to perform their gliding motions. In the proposed ‘capping’ model, an uncharacterized protein complex moves rearward, moving the parasites forward.
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