OKAMURA, BETH; CURRY, ALAN; WOOD, TIMOTHY S.; CANNING, ELIZABETH U.: Worms to sacs: Buddenbrockia reveals bilaterian origins of the Myxozoa

Buddenbrockia is an enigmatic, worm-like parasite of freshwater bryozoans which was first described in the 19th century. It has since been reported infrequently and its phylogenetic affinities have remained obscure. The discovery of Buddenbrockia parasitic in bryozoans from Europe and North America has provided material for the first ultrastructural study which has confirmed the lack of a gut, the presence of an inner cell layer in the body wall, and four sets of longitudinal muscles. The latter feature has long suggested a nematode affinity. However, the possession of polar capsules, places Buddenbrockia in the Phylum Myxozoa. Other features further testify to its myxozoan nature and identify it as a member of the Class Malacosporea. The Myxozoa comprises a group of endoparasites of fish and invertebrates whose higher level phylogenetic affinities are controversial. Our studies have simultaneously revealed that Buddenbrockia is a myxozoan and that the myxozoans are derived from bilaterians, thereby resolving the debate over the triploblastic versus diploblastic nature of the Myxozoa. Our studies also provide evidence that bryozoans are ancestral hosts for the myxozoans and that loss of triploblast features has characterised the major radiation of the better known endoparasites of fish and worms in the Class Myxosporea.