Proliferative kidney disease (PKD), a condition associated with high mortality in salmonid fish, represents an abnormal immune response to the presence of an enigmatic myxozoan, which has been designated simply as PKX organism because its generic and specific status are obscure. Phylogenetic analyses of partial sequences of the 18S rDNA of PKX and of myxozoan parasites infecting the bryozoans Cristatella mucedo, Pectinatella magnifica and Plumatella rugosa, including the previously named Tetracapsula bryozoides from C. mucedo, showed that these taxa represent a distinct clade that diverged early 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 occur in T. bryozoides and PKX but not in the myxozoans belonging to the established orders Bivalvulida and Multivalvulida. Variation of 0·5–1·1% was found 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 distinct taxon, not closely related to PKX but some sequences from myxozoans infecting 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 analysed further, were also found to be phylogenetically indistinguishable from PKX. We propose that at least some variants of these new myxozoans from bryozoans are able to infect and multiply in salmonid fish, in which they stimulate the immune reaction and cause PKD but are unable to form mature spores to complete their development.
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