Molecular data implicate bryozoans as hosts for PKX (Phylum Myxozoa) and identify a clade of bryozoan parasites within the Myxozoa

Proliferative kidney disease (PKD), a condition associated with high mortal ity in salmonid fish, represents an abnormal immune response to the presenc e of an enigmatic myxozoan, which has been designated simply as PKX organis m because its generic and specific status are obscure. Phylogenetic analyse s of partial sequences of the 18S rDNA of PKX and of myxozoan parasites inf ecting the bryozoans Cristatella mucedo, Pectinatella magnifica and Plumate lla rugosa, including the previously named Tetracapsula bryozoides from C. mucedo, showed that these taxa represent a distinct clade that diverged ear ly in the evolution of the Myxozoa before the radiation of the other known myxozoan genera. A common feature of the myxozoans in this clade may be the electron-dense sporoplasmosomes with a lucent bar-like structure, which oc cur in T. bryozoides and PKX but not in the myxozoans belonging to the esta blished orders Bivalvulida and Multivalvulida. Variation of 0.5-1.1% was fo und among the PKX 18S rDNA sequences obtained from fish from North America and Europe. The 18S rDNA sequence for T. bryozoides showed that it is a dis tinct taxon, not closely related to PKX but some sequences from myxozoans i nfecting 2 of the bryozoan species were so similar to those of PKX as to be indistinguishable. Other sequences from the new myxozoans in bryozoans at first appeared distinct from PKX in a maximum likelihood tree but, when ana lysed further, were also found to be phylogenetically indistinguishable fro m PKS. We propose that at least some variants of these new myxozoans from b ryozoans are able to infect and multiply in salmonid fish, in which they st imulate the immune reaction and cause PKD but are unable to form mature spo res to complete their development.