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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, gregarines are 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 treatment options for toxoplasmosis and malaria. ==Lifecycle== 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. Gregarines have 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 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Gregarinasina」の詳細全文を読む スポンサード リンク
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