ULTRASTRUCTURAL-STUDY AND DESCRIPTION OF PERNICIVESICULA-GRACILIS GEN ET SP-NOV (MICROSPORA, PEREZIIDAE), A ROD-SHAPED MICROSPORIDIUM OF MIDGE LARVAE, PENTANEURELLA SP (DIPTERA, CHIRONOMIDAE), IN SWEDEN

The microsporidium Pernicivesicula gracilis gen. et sp. nov., a parasite of larvae of the midge Pentaneurella sp. in Sweden, is described based on light microscopic and ultrastructural characteristics. Merogony is diplokaryotic. Each diplokaryon of the merogonial plasmodium matures to a sporont diplokaryon, and the plasmodium thus matures into a sporogonial plasmodium which divides by rosette-like budding. The number of lobes equals the number of diplokarya. Sporogony comprises meiotic and mitotic divisions, and each diplokaryon yields a chain of eight sporoblasts. A sporophorous vesicle and a parasitophorous vacuole enclose the total amount of spores produced by the plasmodium. The monokaryotic spores are rod-shaped, c. 1 x 14 mum in living condition. The thin spore wall has a layered exospore with an internal double-layer. The polaroplast has an anterior lamellar and a posterior sac-like part. The isofilar polar filament is straight without coils. Young sporophorous vesicles have granular inclusions, older vesicles have tubules of exospore origin. The microsporidium is compared to the microsporidia of midge larvae, and the taxonomy on genus and family levels is briefly discussed. The new genus is provisionally included in the family Pereziidae.