Molecular phylogeny of microsporidians with particular reference to species that infect the muscles of fish.

Ribosomal DNA from eight species of microsporidians infecting fish have been sequenced. Seven of these species infect the skeletal muscle of fish (Pleistophora spp.) and one species infects migratory mesenchyma cells (Glugea anomala). These sequences, in addition to other available microsporidian rDNA sequences from a broad range of host taxa, have been used in phylogenetic analysis. This analysis revealed that muscle-infecting microsporidians from fish are a polyphyletic group, indicating that characters supposed to be important in the classification of the genus Pleistophora have to be re-evaluated. One character that probably has a polyphyletic origin is the amorphous coat, which has been extensively used in the definition of this genus. Furthermore, our results showed that the insect parasitizing Pleistophora spp. are not related to the true pleistophorans parasitic in skeletal muscle of fish. Phylogenetic analysis of small subunit rDNA sequences revealed disagreements between the molecular phylogeny and classifications based upon ultrastructure. Many of the morphological characters claimed to be important in microsporidian classifications appeared to have arisen several times during evolution: for example, the diplokaryon and sporophorous vesicles.