Relationships among members of the genus Myxobolus (Myxozoa : Bilvalvidae)based on small subunit ribosomal DNA sequences

Sequences representing similar to 1,700 base pairs of the 18S rRNA gene fro m 10 different species in the genus Myxobulus were found to group them into 3 clusters that showed little correlation with spore morphology and size o r host specificity, criteria currently used for both higher and lower taxon omic placements in the Myxozoa. Of the phenotypic criteria examined, tissue tropism was most correlated with the rRNA groupings observed. Spores of si milar size and shape (Myxobolus cerebralis vs. Myxobolus squamalis) were di stantly related in some instances, whereas spores with divergent morphology and size were some times found to be closely related (M. cerebralis and My xobolus insidiosus). These initial investigations into the phylogenetic rel ationships of putative members of the genus Myxobolus clearly indicate the potential limitations of groupings based on size and morphological properti es of the spores and host species infected. We propose that 18S rRNA gene s equences, combined with information on tissue tropism, host species infecte d, and developmental cycles in the fish and alternate host (when and if kno wn) be given greater consideration in taxonomic placements of myxosporeans.