Microsporida

Intracellular parasites, no mitochondria, ribosomes are unusual in being of prokaryotic size (70S) and lacking characteristic eukaryotic 5.8S r RNA as a separate molecule in the microsporidia but is incorporated into the 23S r RNA; comparison of rRNA genes suggests that microsporidia are ancient eukaryotes; comparison of tubulin gene sequences suggest that they are related to fungi; hosts include most invertebrate phyla; all classes of vertebrates, the greatest number of species being known from arthropods and fish; with growing and dividing stages (meronts and sporonts), and spores which are used for transmission between hosts; meronts with one nucleus or two closely adhering and synchronously dividing nuclei; with endoplasmic reticulum, ribosomes and an atypical dictyosome but no mitochondria, flagella, or cytoskeletal structures; sporonts have more abundant endoplasmic reticulum and develop a surface coat which becomes the outer layer of the spore wall; spores unicellular with one or two nuclei, a polar tube (polar filament), the polaroplast and the posterior vacuole; cytoplasm and nucleus (or nuclei) become the infective agent (sporoplasm), as it emerges from the spore; meronts, ranging from small rounded cells to plasmodia or ribbon-like formations, divide repeatedly by binary fission, plasmotomy or multiple fission; merogony is followed by sporogony, in which cells known as sporonts are committed to spore production; sporonts, divide into sporoblasts, the number of which is characteristic of the genera; sporoblasts mature into spores; but individual life cycles are highly variable; meiosis occurs and this indicates that gametogenesis and fusion of gametes must occur but this has been recognised for only a few species; genera with an alternation of diplokaryotic and monokaryotic stages can be dimorphic and heterosporous. Genus descriptions are usually based on the type species.