Phylogenetics: Taxonomy and the microsporidia as derived fungi

Abstract

Microsporidia are a group of obligate eukaryotic intracellular parasites first recognized over 100 years ago with the description of Nosema bombycis the parasite from silkworms that caused the disease pebrine in these economically important insects. Microsporidia infect almost all animal phyla. Among the more than 144 described genera, several have been demonstrated in human disease: Nosema, Vittaforma, Brachiola, Pleistophora, Encephalitozoon, Enterocytozoon, Septata (reclassified to Encephalitozoon) and Trachipleistophora. In addition, the genus Microsporidium has been used to designate microsporidia of uncertain taxonomic status. The recognition of microsporidia as opportunistic pathogens in humans has led to increased interest in the molecular biology of these pathogens. Recent work has focused on the determination of the nucleotide sequences for ribosomal RNA (rRNA) genes, which have been used as diagnostic tools for species identification as well as for the development of a molecular phylogeny of these organisms. Microsporidia have historically been considered to be ``primitive'' protozoa, however, molecular phylogenetic analysis has led to the recognition that these organisms are not ``primitive'' but degenerate and that they are related to the fungi and not to other protozoa. Such molecular phylogeny has also led to the recognition that the traditional phylogeny of these organisms based on structural observations may not reflect the ``true'' relationships among the various microsporidia species and genera. This chapter reviews the data on the taxonomy of the microsporidia and the relationship of these organisms to other eukaryotes.