Gregarinasina
| Gregarines | |
|---|---|
| Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Chromalveolata |
| Superphylum: | Alveolata |
| Phylum: | Apicomplexa |
| Class: | Conoidasida |
| Subclass: | Gregarinasina |
| Orders | |
The gregarines are a group of Apicomplexan protozoa, classified as the Gregarinasina[1] or Gregarinia.
The large (roughly half a millimeter) parasites inhabit the intestines of a large number of invertebrates. They are not found in humans. 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 to invade target cells.[2][3] This makes them an excellent model for studying gliding motility with the goal of developing toxoplasmosis and malaria treatment options.
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[edit] Life cycle
Gregarines occur in both aquatic and terrestrial environments. Althought 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, 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.
[edit] Taxonomy
The Gregarines are separated into three taxa based on habitat, host range and trophozoite morphology: archigregarines, eugregarines and neogregarines.
Archigregarines are found only in marine habitats. They possess intestinal trophozoites that are similar in morphology to the infective sporozoites.
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. Intestinal eugregarines are separated into septate and aspetate (which are mostly marine) gregarines depending on whether the trophozoite is superficially divided by a transverse septum.
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 annelids. These latter species tend to branch closely with neogregarines and may need to be reclassified.
Neogregarines are found only in terrestrial hosts. These species have reduced trophozoites and tend to infect tissues other than the intestine.
DNA studies suggest that the archigregarines are ancestral to the others. The eugregarines comprise the majority of the known species.
[edit] Molecular biology
The gregarines are able to move and change direction along a surface through gliding motility without the use of cilia, flagella, or lamellipodia.[4] This is accomplished through the use of an actin and myosin complex.[5] The complexes require an actin cytoskeleton to perform their gliding motions.[6] In the proposed ‘capping’ model, an uncharacterized protein complex moves rearward, moving the parasites forward.[7]
[edit] References
- ^ Carreno RA, Martin DS, Barta JR (November 1999). "Cryptosporidium is more closely related to the gregarines than to coccidia as shown by phylogenetic analysis of apicomplexan parasites inferred using small-subunit ribosomal RNA gene sequences". Parasitol. Res. 85 (11): 899–904. doi:10.1007/s004360050655. PMID 10540950. http://link.springer.de/link/service/journals/00436/bibs/9085011/90850899.htm.
- ^ Menard, R. 2001. Gliding Motility and Cell Invasion by Apicomplexa: Insights from the Plasmodium sporozoite. Cell. Microbiol 3: 63-73.
- ^ Meissner, M, Schluter, D and Soldati, D. 2002. Role of Toxoplasma Gondii Myosin a in Powering Parasite Gliding and Host Cell Invasion. Science 298: 837-841.
- ^ Walker, M M., C Mackenzie, S P Bainbridge, and C Orme. 1979. A Study of the Structure and Gliding Movement of Gregarina garnhami. J Protozool 26: 566-574.
- ^ Heintzelman, M B. 2004. Actin and Myosin in Gregarina polymorpha. Cell Motil Cytoskeleton 58:83-95.
- ^ Mitchison, T.J., and Cramer, L.P. 1996. Actin-based cell motility and cell locomotion. Cell 84: 371–379
- ^ Sibley, L.D., Hakansson, S, Carruthers, V. B. 1998. Gliding motility: An efficient mechanism for cell penetration Curr. Biol. 8, 12.
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