Organization of frozen-etched Thelohania bracteata (Strickland, 1913) (Microsporida, Nosematidae) emphasizing the fine structure of the posterior vacuole

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Abstract
This study examines the ultrastructure of the posterior vacuole in frozen-etched spores of Thelohania bracteata (Nosematidae), a microsporidian infecting the fat body of larval black-flies (Simuliidae). This organelle, considered important in providing intrasporal pressure for sporoplasm extrusion through the polar filament, has a double membrane with particles on its internal faces as revealed by the freeze-etching technique. The size and pattern of these particles differ from those in membranes of the polar filament and nucleus, and this difference may have functional significance. The posterior vacuole, and also the polaroplast, may originate from expanded sacs that occur in the immature spore. There is evidence from this study that there are many basic ultrastructural similarities between spores of different microsporidian species and that at least some reported differences are the result of varying techniques.We gratefully acknowledge the freeze-etching facilities provided by the Department of Pathology, Faculty of Medicine, McMaster University. The research was supported by Grant A-130 from the National Research Council of Canada.