Taxonomy

Microsporidia have had a long history of being classified and reorganized into various groupings based on classical morphological studies and more recent phylogentic analyses. Nägeli (1857) placed the newly named Nosema bombycis within the group Schizomycetes, which at that time consisted of mostly yeast and bacteria. To make a clearer distinction between the Nosema sp. and other organisms in this group, in 1882 the order Microsporida was created (Franzen and Muller, 1999a and Didier et al., 2004).

During the 1900’s, several classification systems were devised based on ultrastructural studies, number of nuclei in each organism, spore morphology, and differences in developmental stages (Didier et al., 2004).  However until recently, the most widely accepted classification proposed by Sprague (1976) was based on membrane delimited or direct cytoplasmic development of the parasites.

This system was again modified to incorporate the nuclear state as the primary defining characteristic, such as diplokaryon or haplokaryon, in organizing specific species of microsporidia (Franzen and Muller, 1999a).

While microsporidia have been classified as eukaryotic protozoa, new phylogenetic studies suggest these organisms are more closely related to fungi, and therefore should be reclassified as such (Thomarat et al., 2004).

Initial phylogenetic studies of ribosomal RNA and elongation factors would argue that these organisms were primitive eukaryotes that diverged early in history, prior to gaining the endosymbiont α-proteobacteria and should be placed with Archezoa (Gill and Fast, 2006).

later studies that focused on the genes of α- and β-tubulin, mitochondrial heat-shock protein 70, TATA-box binding protein, the large subunit of RNA polymerase II, and subunits of pyruvate dehydrogenase proposed a stronger relationship between microsporidia and fungi (Gill and Fast, 2006).

Structural and physiological similarities between fungi and microsporidia have been noted, such as the abilities to form spores and synthesize chitin, a component of the spore coat (Thomarat et al., 2004).

The structural, biological, and recent phylogenetic similarities between microsporidia and fungi have strengthened the idea that these organisms are related to fungi; however, the exact placement within this kingdom has not been resolved (Gill and Fast, 2006).

 Current studies have placed them with the zygomycetes (Keeling, 2003) and chytrids (James et al., 2006), while another report suggests that they are a sister to a combined basdiomycetes/ascomycetes group (Gill and Fast, 2006).

In conclusion, 150 years of research on microsporidia evolution has come full circle: they began as fungi for misguided reasons, and made their way through various groups of protozoa, and now the microsporidia are considered to be highly-diverged, well-adapted, and specialized parasites that are related or belong to the fungi or perhaps represent a sister group to the fungi. Additional studies must be completed to identify the complete taxonomic placement of these eukaryotes (Gill and Fast, 2006 and Redhead et al., 2009).