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The microsporidia constitute a phylum (Microspora) of spore-forming unicellular parasites. They were once thought to be protists but are now known to be fungi. Loosely 1500 of the probably more than one million species are named now. Microsporidia are restricted to animal hosts, and all major groups of animals host microsporidia. Most infect insects, but they are also responsible for common diseases of crustaceans and fish. The named species of microsporidia usually infect one host species or a group of closely related taxa. Several species, most of which are opportunistic, also infect humans. Approximately 10 percent of the species are parasites of vertebrates — including humans, in which they can cause microsporidiosis. After infection they influence their hosts in various ways and all organs and tissues are invaded, though generally by different species of microsporidia. Some species are lethal, and a few are used in biological control of insect pests. Parasitic castration, gigantism, or change of host sex are all potential effects of microsporidian parasitism (in insects). In the most advanced cases of parasitism the microsporidium rules the host cell completely and controls its metabolism and reproduction, forming a xenoma. .〔Ronny Larsson, Lund University (Department of Cell and Organism Biology) (''Cytology and taxonomy of the microsporidia'' ) 2004.〕 Replication takes place within the host's cells, which are infected by means of unicellular spores. These vary from 1-40 μm, making them some of the smallest eukaryotes. Microsporidia that infect mammals are 1.0-4.0 μm. They also have the smallest eukaryotic genomes. Microsporidium was once the vernacular name for a member of the class Microsporidea in the protozoan subphylum Cnidospora. == Morphology == Microsporidia lack mitochondria, instead possessing mitosomes. They also lack motile structures, such as flagella. Microsporidia produce highly resistant spores, capable of surviving outside their host for up to several years. Spore morphology is useful in distinguishing between different species. Spores of most species are oval or pyriform, but rod-shaped or spherical spores are not unusual. A few genera produce spores of unique shape for the genus. The spore is protected by a wall, consisting of three layers: *an outer electron-dense ''exospore'' *a median, wide and seemingly structureless ''endospore'', containing chitin *a thin internal ''plasma membrane'' In most cases there are two closely associated nuclei, forming a ''diplokaryon'', but sometimes there is only one. The anterior half of the spore contains a harpoon-like apparatus with a long, thread-like ''polar filament'', which is coiled up in the posterior half of the spore. The anterior part of the polar filament is surrounded by a ''polaroplast'', a lamella of membranes. Behind the polar filament, there is a posterior ''vacuole''.〔 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「Microsporidia」の詳細全文を読む スポンサード リンク
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