Author: Jan Scholten
Names: Microsporidiomycota; Microsporidium; Microsporidia; Microsporidian.
Genera: 1500 species are named, of the probably more than one million existing.
Microsporidia were once considered protozoans or protists. They are now seen as fungi or a sister group to fungi.
Microsporidia are the smallest eurakyota. They are spore-forming unicellular parasites of insects, crustaceans and fish. They can infect immune suppressed humans, invading various organs and tissues, some lethal. They are used in biological control of insect pests, parasitic castration, gigantism, or change of host sex, sometimes ruling the host cell completely, forming a xenoma. Replication takes place within the host's cells, infected by unicellular spores, varying from 1 to 40 μm. They lack mitochondria, instead possessing mitosomes. They lack motile structures, such as flagella. Their spores are highly resistant, surviving up to several years, oval or pyriform, but rod-shaped or spherical spores are not unusual, 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. They have two closely associated nuclei, forming a diplokaryon. 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.
In the gut of the host the spore germinates, it builds up osmotic pressure until its rigid wall ruptures at its thinnest point at the apex. The posterior vacuole swells, forcing the polar filament to rapidly eject the infectious content into the cytoplasm of the potential host. Simultaneously the material of the filament is rearranged to form a tube which functions as a hypodermic needle and penetrates the gut epithelium.
Once inside the host cell, a sporoplasm grows, dividing or forming a multinucleate plasmodium, before producing new spores. The life cycle varies considerably. Some have a simple asexual life cycle, while others have a complex life cycle involving multiple hosts and both asexual and sexual reproduction. Different types of spores may be produced at different stages, probably with different functions including autoinfection (transmission within a single host).