ABSTRACT
Microsporidia are evolutionally ancient organisms that display features reminiscent of both prokaryotes and eukaryotes. On the one hand, microsporidia possess a nucleus, mitotic spindle-separated chromosome, an intracytoplasmatic membrane system (cytoskeleton), and polyadenylation on mRNA, typical of eukaryotes. On the other hand, microsporidia have distinct translational apparatus and small genome size but lack recognizable mitochondria, peroxisomes, and a classical stacked Golgi apparatus. In addition, microsporidial ribosomes (70S), ribosomal subunits (30S and 50S), and rRNA (16S and 23S) are characteristic of prokaryotes, with the absence of distinct 5.8S rRNA.1,2 Interestingly, sequence examination of microsporidian small-subunit (SSU) rRNA suggested that microsporidia may have branched off prokaryotes on the way to become eukaryotes. However, phylogenetic trees constructed on the basis of α-and β-tubulin as well as Hsp70 (a heat-shock protein or chaperonin) gene sequences and those of other genes indicated that the microsporidia are close relatives of fungi and may have evolved degeneratively from higher forms.3-5
Not surprisingly, considerable uncertainty has existed in relation to the taxonomic status of microsporidia. For a
long period, microsporidia have been classi–ed under the phylum Archezoa and subkingdom Protozoa, which covers primitive amitochondriate eukaryotes such as Giardia and Entamoeba. These organisms may have acquired mitochondria by endosymbiosis. Following the whole genome sequence analysis of a representative microsporidial organism (i.e., Encephalitozoon cuniculi), it became apparent that microsporidia have retained mitochondrion-derived doublemembrane organelles, mitosomes, with some mitochondrial functions. Further phylogenetic analyses uncovered that microsporidia are most closely related to the zygomycetes, and their association with fungi is supported by their possession of chitin and trehalose, similar cell cycles, and gene organization of certain genes that are typical of fungi.6-8 Thus, microsporidia are now regarded as a separate phylum in the Fungi kingdom, consisting of two groups, on the basis of the presence or absence of a membrane surrounding the organisms: the Pansporoblastina (membrane present) and the Apansporoblastina (membrane absent). This may have implications for the discovery of new therapeutic strategies.9
