The fine structure of the spore of Nosema spelotremae, a microsporidian parasite of a Spelotrema metacercaria encysted in the crab Carcinus maenas

Abstract

The fine structure of the spore of Nosema spelotremae was studied by electron microscopy. The spores are 2.6–3.0 μ by 1.3–1.5 μ in size, and the polar filament is about 106 μ long. The general structure resembles that found by Huger for the spore of Nosema locustae. The polar filament, which varies in diameter between about 90 and 110 mμ, is coiled regularly at the posterior end of the spore, usually in single but sometimes in double or triple coils. At the anterior end the coils are irregular, and a straight section runs through the polaroplast towards the point of attachment near the anterior pole. At the attachment point the filament is anchor-shaped, and the arms of the anchor extend backwards, narrowing to a diameter of 15 mμ. The filament is double-walled, and there is some evidence that it contains a core of solid material, but no regular arrangement of fibrils was seen insede the filament. The polaroplast contains laminae with a repeat distance of about 50 Å. No clear nucleus is seen, but a loose-textured rather electron-transparent area lying inside the regularly coiled part of the polar filament at the posterior end of the spore is considered to be nuclear material. The spore coat comprises, on the outside, two electron-dense layers containing a less dense layer, the whole being about 25 mμ thick. Inside this is an electron-transparent layer 50 mμ thick, bounded on its inner side by a narrow electron-dense layer, which is separated by a narrow gap from the spore membrane. There is a slight thinning of the spore coat at the point of anchorage of the polar filament, this being at the expense of the wide electron-transparent layer.