INTRODUCTION TO THE MICROSPORIDIA

The microsporidians are intracellular parasites of animals and protists. They are highly reduced in form and develop in intimate association with certain parts of cells or organelles (e.g. Chytridiopsis grows within the host nucleus; Figure 1). They can be quite important, especially to those who raise bees and fish. Glugea (Figure 2) attacks fish and makes large tumor-like cysts on their gills, bodies, and organs. Nosema (Figure 3) can infect honeybee colonies, particularly in the spring and decimate them. The everted polar filament of Nosema in Figure 3 and 4d-f would inject the sporoplasm into a target cell. There, the injected cell develops into a small plasmodium, which then differentiates into many infective spores (Figure 4).

FIGURE 1. Spores of Chytridiopsis developing within the host nucleus. Image from http://www.biol.lu.se/cellorgbiol/microsporidia/9sm_chytridiopsis.jpg	FIGURE 2. Living spores of Glugea with the coiled polar filament visible in some of the spores. Image from http://www.biol.lu.se/cellorgbiol/microsporidia/9sm_chytridiopsis.jpg	FIGURE 3. A spore of Nosema with the polar filament (text with tooltip) Cellular organelle coiled inside spore. When a spore encounters a new host cell, the filament uncoils and injects the infective sporoplasm through the filament. everted. Image from http://www.biol.lu.se/cellorgbiol/microsporidia/9sm_chytridiopsis.jpg

FIGURE 4. Nosema Life Cycle in Apis, the honeybee. a&b the spore is ingested and the thread everts to release the sporoplasm (c-f). It grows as an intracellular parasite (g-m) and then begins to produce spores (n-q). The spores are released into the gut and pass out of the body in the feces (r&t). A mature spore (v). Image from Olsen 1974.

About 14 taxa of microsporidians, primarily species of Encephalitozoon, are human pathogens. They infect the large and small bowel, the kidneys, the eyes, muscles, and sinuses. They are spread primarily by inhalation or ingestion.

Not all of them have been detrimental. Some offer the promise of pest control without pesticides. Because they are somewhat specific as parasites, they have been tried as biological controls for corn borer, crickets, grasshoppers, spider mites, webworms, and cabbage looper. The results, though hopeful, have been mixed.

The microsporidians have been taxonomic nomads. They once were considered to be protozoans (animals), then protists without taxonomic affiliations. The relationship with the fungi was slow in coming. Keeling (2002), based on a molecular analysis, places them within the Zygomycota. However, because the zygomycotes likely are paraphyletic, their taxonomic structure will be modified. So, the wanderings in the taxonomic wilderness are not over for the microsporidians.

FIGURE 5. A cladogram showing the relationships between Microsporidian classes (taxa in the shaded box) and the other fungal taxa. The topology is supported by James et al. (2006).

SYSTEMATICS OF THE MICROSPORIDIA

Patterson (1999) identifies their defining synapomorphy as an introverted filament, and thus considers them to be a natural group. However, until recently their affinities were a mystery.

Generally, the microsporideans have been considered “sporozoans” (e.g. Grell 1973; and Kudo 1966). As recently as 1988, Margulis and Schwartz lumped the microsporidians and the myxozoans together in the phylum, Cnidosporidia (Pr-20), but they later separated them (Margulis and Schwartz 1998; Pr-2). Weiser (1985b) was one of the first students of the group to recognize their unique features and push to give them phylum-level status. Those who follow the Archezoa Hypothesis (e.g. Cavalier-Smith, 1983; and Tudge 2000) consider them to be primitive and branch from the base of the Eukaryotic tree. However, recent evidence (Fast and Keeling 2001; Keeling 2002; Sokolova et al. 2003; and Ragan et al. 2003) places them squarely in the opisthokonts, allied with the fungi. Indeed, Keeling (2002) seems to show adequate evidence that the microsporidians are parasites that evolved from a zygomycete ancestor. Gryzanskyi et al. (2013) suggest that the Entomophthoromycota, a phylum of conjugating fungi, might be oldest extant lineage of non-flagellated fungi and may be the closest relative of the Microsporidia. The most recent taxonomic system for the microsporidians is that of Sprague et al. (1992), who proposed a complex scheme with three classes based entirely on morphology. Canning and Vavra (2000) back away from the use of all higher taxa within the phylum as do Adl et al. (2005 and 2012). Thus, I will continue to use the more traditional morphological taxonomic system of Weiser (1985b) until a usable system confirmed by molecular evidence develops.