Life cycles

Complete life cycles have been elucidated for only few microsporidian species. At present, life cycles comprising one or two host species are known, and up to three morphologically

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different types of spores are produced in different parts of the cycle. It seems that most microsporidia complete their life cycle in one host and only one spore type is produced. Life cycles involving alternate hosts are so far restricted to mosquitoes and copepods (Becnel 1994). Life cycles of vertebrate microsporidia appear comparatively simple, and no involvement of alternate hosts is known. In an increasing number of cases of microsporidian infections of insects it has been revealed that the life cycle involves two sequences of sporogony in the same host, yielding two kinds of slightly different spores (Iwano and Ishihara 1991). The first sporogony occurs in the early phase of infection, yielding spores that differ from the second type of spores in that the endospore layer is weakly developed and the polar filament is shorter, which are typical signs of the spores being immature. These spores germinate in the cell where they have been produced, and their function is to facilitate the dispersal of the microsporidia inside the host where they were produced. The second type of spore, which is completely mature, is produced in a different tissue. This is the infectious stage for new hosts.

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