ABSTRACT
Microsporidia are strictly intracellular, eukaryotic parasites characterized by unicellular spores containing one sporoplasm and an elaborate extrusion apparatus serving to inject the infective germ, the sporoplasm, through a hollow polar tube into the cell of a new host. The proliferative stages may be multinucleate and possess a very primitive structure; true mitochondria are missing throughout the life cycle. Food uptake is accomplished by membrane transport. Many of the genera parasitic in fish enter a complicated coexistence with their host cell, inducing a special type of hypertrophy and turning it into a structure called xenoma. Early analysis of the SSU rDNA placed microsporidia at the base of the eukaryote phylogenetic tree. Recent research indicates, however, that they are close to fungi or even belong to fungi. They have the smallest genomes among eukaryotes (in two fish microsporidian species examined thus far, it is 6.2 and 19.5 Mbp), prokaryote-like ribosomes and they miss
reticulum, free ribosomes, various vesicles of undetermined nature and mitosomes, i.e., remnants of the once present mitochondria (Williams and Keeling, 2005). In most fish microsporidia, all life cycle stages have a single, isolated nucleus; a diplokaryon is an exception. Nuclear division is a cryptomitosis, taking place within an intact nuclear envelope (Fig. 7.9). Meronts lie mostly in direct contact with the host cell cytoplasm, sometimes within an envelope of host rough endoplasmic reticulum. An exception is the genus Heterosporis, in which there is a thick envelope around all developmental stages, growing along with proliferation of the parasite. As a rule, meronts have a thin plasmalemma, but in Pleistophora they have a thick envelope. There may be one or several generations of meronts. Multinucleate meronts may reach up to 30 |im.
