Microsporidia are eukaryotic organisms, producing spores of unique construction. All life cycle stages are intracellular. The cytology is of normal eukaryotic type with a few important specializations (Vavra 1976; Larsson 1986; Vavra and Larsson 1999). All life cycle stages lack mitochondria and the nuclei are either isolated or coupled as diplokarya. Further, the RNA is of prokaryotic type (Curgy et al. 1980;

Vossbrinck et al. 1987). While presporal stages, even to the specialist, are difficult to identify as microsporidia, the spores, which have a thick and complex spore wall and a unique infection apparatus, are so characteristic that with basic knowledge of the group it is normally no problem to recognize their microsporidian nature.

The infectious stage is the spore. Living spores of most microsporidia range in size between 1 and 5^m, but a small number of species produce larger spores (up to 40^m). The spore shape is, for most species, oval or pyriform, less commonly rod-like or spherical (Figure 11.1A-D). A small number of genera, like Cougourdella and Caudospora, have spores of unique shape for the genus. It is often possible to identify microsporidia to a genus using the spore shape in combination with the way the spores are produced (Larsson 1983b, 1988, 1999).

The spore wall is normally composed of three layers: an external more or less complex exospore, a median wide, seemingly structureless endospore containing chitin (Vavra 1976), and the internal plasma membrane (Figure 11.1C).

The infection apparatus is composed of three components. The anterior half of the spore is occupied by a system of lamellae or sacs delimited by unit membranes (the polaroplast) (Figure 11.1C). The polaroplast is normally divided into two, or sometimes three, structurally different parts. It surrounds the anterior part of a long thread-like organelle (the polar filament) which is attached to an anchoring apparatus at the anterior pole of the spore. The filament proceeds straight backwards through the centre of the spore, approximately to the middle of the spore, while the posterior part usually is coiled up in one, or sometimes more, layers of coils in the posterior half of the spore (Figure 11.1C). The filament is composed of concentric layers of different electron density, and it is externally covered by a unit membrane which is continuous with the membrane component of the polaroplast. The width of the filament is either uniform from the anterior to the posterior end (isofilar filament) or the anterior part of the filament is wider (anisofilar filament, Figure 11.1C). All species belonging to the same genus have either isofilar or anisofilar filament. The third component of the extrusion apparatus is the posterior vacuole, a membrane-lined cavity at the posterior end of the spore.

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