Ultrastructural study and description of Flabelliforma magnivora sp. n. (Microspora: Duboscqiidae), a microsporidian parasite of Daphnia magna (Crustacea: Cladocera: Daphniidae)

The new microsporidium Flabelliforma magnivora sp. n. is described based primarily on ultrastructural characteristics. All life cycle stages have isolated nuclei. Merogonial and sporogonial reproductions are by rosette- or finger-like budding. The sporogony yields 4-16, most frequently 12, sporoblasts. A fragile sporophorous vesicle is formed by the sporont. Spores are lightly pyriform, with both poles blunt, often with one surface slightly convex. Unfixed spores measure 2.34-3.03 x 4.07-4.93 μm. The exospore, which is five-layered with an internal double-layer, measures 35-40 nm thick. The polar filament is isofilar with 14-17, 77-94 nm wide coils arranged in two irregular layers of coils in the posterior half of the spore. The polaroplast, which ends close to the anterior polar filament coils, is composed of two regions of regularly arranged lamellae: narrow lamellae anteriorly, wider lamellae posteriorly. The host is the cladoceran Daphnia magna. The primary site of infection is the fat body, but infection was also observed in the hypodermic cells and the ovaries. In the laboratory the parasites are transmitted with nearly 100% fidelity from mother to offspring. Transversely sectioned polyribosomes appeared as characteristic circular configurations of 9 ribosomes. Teratological development, including incomplete separation of sporoblasts, macrospores with supernumerary polar filament coils, and disturbed coiling, was observed. The identification of the species and the genus position are briefly discussed.