Two new species of Microsporidia were recognized in skeletal muscle of freshwater fishes from Finland. Myosporidium spraguei n. sp. from pike-perch Sander lucioperca occurred as mature spores within sporophorous vesicles (SPVs) within a xenoma. The ovoid spores were 3.8 μm long and 2.4 μm wide, based on transmission electron micrographs (TEM). The exospore and endospore were equally thick, the nucleus was monokaryotic and the polar filament was isofilar with 12 coils in a single rank, entirely adjacent to the prominent posterior vacuole. Small subunit (SSU) rDNA sequence confirmed the presence of M. spraguei n. sp. in burbot Lota lota. The second species, Microsporidium luciopercae n. sp., also from pike-perch, occurred within SPVs that occupied only a fraction of the volume of the otherwise intact myocyte; no xenoma was produced. Myocyte degeneration and necrosis occurred as mature spores dispersed into direct contact with the sarcoplasm. The ovoid spores were 4.6 μm long and 2.8 μm wide (based on TEM); they were monokaryotic and the polar filament was isofilar with 25 coils in a single rank in the posterior of the spore. The exospore was relatively thin with an irregular profile. Neither infection elicited an inflammatory response, although degenerate spores were observed within host cells, suggesting phagocytosis. Phylogenetic analysis of SSU sequences placed both organisms on distinct clades within the Marinosporidia.

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