Developmental cycle and ultrastructure of Steinhausia mytilovum, a microsporidian parasite of oocytes of the mussel, Mytilus galloprovincialis (mollusca, bivalvia)

Abstract

Steinhausia mytilovum was found parasitizing the oocytes of mussels, Mytilus galloprovincialis in a natural population in the western Mediterranean, near Barcelona (Spain), Optical, confocal and transmission electron microscopy revealed that this microsporidian developed within a vacuole in the cytoplasm or the nucleus of oocytes. Each vacuole contains a variable number of Steinhausia cells at the same stage of maturation. Ultrastructural studies allowed examination of developmental stages: merogonial plasmodium, sporonts, sporoblasts, prespores and spores. The merogonial plasmodium showed endomitosis with a centriolar plaque and intranuclear microtubules. Sporoblasts were diplokaryotic, and round or oval. There was an incipient polar filament in the central region. The prespores, also diplokaryotic, were oval. They showed an electron dense tubular system connected to the polar filament in formation. Mature spores had diplokarya, an electron dense cytoplasm and a coiled isofilar polar filament in the posterior region. An exact diagnosis of the genus Steinhausia is provided on the basis of ultrastructural data of S. mytilovum.