Reproduction in the host cell

The life cycle of most microsporidia comprises two sequences of proliferation. The first, which is called merogony, starts directly upon infection (Figure 11.1E). It yields daughter cells (merozoites) with the potential either to repeat the merogony or to enter the second phase of reproduction, the sporogony. The daughter cells of the sporogony (sporoblasts) mature to spores without further division. Both cycles of reproduction proceed as binary fission, plasmotomy (splitting of a multinucleate plasmodium), or schizogony (multiple budding), and for each species of microsporidia the modes of reproduction are constant (Figure 11.1E).

Spores are either produced free in the host cell or are enclosed in an envelope, formed by the microsporidium, called a sporophorous vesicle (Figure 11.1B,F). This is a secretory material, most commonly produced by the sporont at the beginning of the sporogony (sporontogenetic sporophorous vesicle). In a few genera of microsporidia, for example Pleistophora and Trachipleistophora, the sporophorous vesicle is initiated already in the merogonial part of the life cycle (merontogenetic sporophorous vesicle). This means that the merontogenetic vesicle not only encloses the daughter cells of one sporont, but also collects the daughter cells of all sporonts produced by one merogonial mother cell. The envelope of a sporophorous vesicle is normally not visible in a light microscopic preparation, but the occurrence of groups of spores in regular numbers indicates the presence of vesicles (Figure 11.1B).

The characteristic organelles of the spore are formed in the sporoblast stage, or in some species, earlier in the sporogony. The sporulating microsporidium covers itself with a more or less complex layer of material outside the plasma membrane. This layer survives as the exospore layer of the spore. With very few exceptions, the polar filament is generated in the sporoblast stage from a vesicular area (Golgi apparatus) close to the nucleus (Vavra 1976). The length of the filament increases until the spore is mature. In Enterocytozoon, and a few more genera, the polar filament is initiated already in the sporogonial plasmodium (Desportes et al. 1983). The endospore layer is the last structure of the spore to form. Consequently, immature spores could be identified as such from the shorter polar filament and the thinner endospore layer.

Sexual processes have been observed in a small number of microsporidia. Zygotes are normally formed at the beginning of the sporogony, and clear cases have been observed in species shifting from diplokaryotic condition in the merogonial part of the life cycle to isolated nuclei in the sporogonial stages and spores. There are varying opinions about the interpretation of the sexual processes (Canning 1988; Flegel and Pasharawipas 1995). Most reports of sexuality in microsporidia are not based on observed zygote production but on the presence of synaptonemal complexes (coupled chromosomes in the reductional division) visible in ultrathin sections of sporonts (Larsson 1986).

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