Rectispora reticulata gen. et sp. nov. (Microspora, Bacillidiidae), a new microsporidian parasite of Pomatothrix hammoniensis (Michaelsen, 1901) (oligochaeta, tubificidae).

The microsporidium Rectispora reticulata gen. et sp. nov., a parasite of male gonads of the oligochaete Pomatothrix hammoniensis in Sweden, is described based on light microscopic and ultrastructural characteristics. Merogonial and sporogonial stages are diplokaryotic. Sporogony is disporoblastic. The spores are cylindrical, often slightly curved and wider in the mid-region, 1.8-2.1 × 8.1-14.2 um long in fixed and stained condition. The spore wall has a ca. 43 mm thick, uniform exospore. The polaroplast has an anterior part with compressed lamellae and a posterior section with 53-128 nm wide tubules. The polar filament has a 181-283 nm wide, straight, short, anterior section in the centre of the spore, an oblique section with tapering diameter, and a long, posterior, 119-178 nm wide section, arranged in (10-)12(-13) isofilar coils, close to the spore wall in the posterior half of the spore. The most anterior and most posterior coils are irregularly arranged, the median coils form a close, regular group. The angle of tilt of the most anterior, regularly arranged coil is ca. 55°. The transversely sectioned filament has distinct concentrical layers of variable thickness and electron density. The diplokaryon occupies the centre of the spore. Sporophorous vesicle absent, but a folded, ca. 16 nm thick envelope, resembling a sporophorous vesicle, is formed by delamination of exospore material. Teratological sporogony was observed. The microsporidium is compared to microsporidia of the genera Bacillidium, Jirovecia and Hrabyeia. The placing of the new genus in the family Bacillidiidae, the distinction of the families Bacillidiidae and Nosematidae, and the various origins of spore-containing envelopes are discussed.