Ultrastructure and localisation of late-sporogonic developmental stages of Sphaerospora ranae (Myxosporea: Sphaerosporidae)

Abstract

Data on external ultrastructure of myxospores and internal ultrastructure of advanced pseudoplasmodia and myxospores of topotypic samples of Sphaerospora ranae (Morelle, 1929) from Rana dalmatina Bonaparte are provided, together with in situ hybridisation results. In both frogs examined, the infection was restricted to renal tubules and corpuscles. The infection site restriction was confirmed by light and transmission electron microscopy, as well as by in situ hybridisation. In addition, large myxospore masses measuring up to 500 microm were detected in seminal vesicles. Only late-sporogonic stages, i.e. pseudoplasmodia harbouring immature and/or mature myxospores, were observed and analysed. Scanning electron microscopy revealed that spores have smooth surface with exception of posterior valvular bulges, which possess numerous outwards opening internal canals. As revealed by both scanning and transmission electron microscopy, the canals are continuous invaginations of the outer spore surface. Myxospores of S. ranae are characterised by the presence of two uninucleate sporoplasms, bilayered polar capsules, S/H-shaped polar filaments in transversal section and multilayered polar filament eversion pole plugging complex. Ultrastructural observations are discussed in the context of available data for other species of Sphaerospora sensu stricto and apparent synchronisation of myxospore shedding with a brief aquatic breeding phase of vertebrate intermediate host is highlighted.