An ancient clade of myxozoans parasitic in freshwater bryozoans

The Myxozoa, an enigmatic group of endoparasites, is marked by a long history of phylogenetic and taxonomic uncertainty. Originally classified as protists, it is now clear that myxozoans are proper metazoans, although controversy remains as to whether they are highly degenerate triploblasts or diploblasts. In addition, taxonomic revision was required when it was discovered that certain myxozoan parasites of worms go on to infect fish, thus uniting within one life cycle, forms that were previously assigned to separate classes. This discovery led to the demise of the Class Actinosporea leaving a single class, the Myxosporea, in the phylum. Recently we have described two myxozoan species that occur as parasitic sacs within the body cavity of freshwater bryozoans (Class Phylactolaemata). Comparisons of morphologies and life cycles have revealed major differences between these bryozoan parasites and the myxozoan parasites of fish and worms. Phylogenetic analysis of 18S rDNA sequences identifies two clades within the Myxozoa that diverged early in the evolution of the group. Our studies suggest that bryozoans may have been ancestral hosts of myxozoans and provide the basis for identifying a new class of myxozoans, the Malacosporea. Finally, our discovery of the bryozoan parasite, Tetracapsula bryosalmonae, has identified the source of the causative agent of Proliferative Kidney Disease, a devastating disease of salmonid fisheries resulting from accidental infection by a previously unknown myxozoan.