Ribosomal DNA Sequence Analysis of Isolates of the PKX Myxosporean and Their Relationship to Members of the Genus

The taxonomy of the PKX myxosporean, cause of proliferative kidney disease (PKD) of salmonids, has long been an enigma because only immature spores are consistently associated with the infection. In the early 1990s, one of us (M.L.K.) proposed that PKX may be the extrasporogonic form of Sphaerospora oncorhynchi, which has been observed in salmonids where PKD is enzootic. Recently, the sequence of the small subunit ribosomal DNA (SSU rDNA) of PKX was deposited in GenBank (U70623). We designed PKX-specific primers by aligning this sequence with that of coho salmon Oncorhynchus kisutch and with those of other myxosporeans. With these primers, a portion of the SSU rDNA of PKX was amplified and sequenced from the kidneys of infected salmonids from British Columbia, Washington, California, Newfoundland, and England. The maximum variation in sequence across 764 base pairs (bp) among all PKX isolates, including the original sequence (GenBank U70623), was 0.8%. Results indicated that PKX isolates from different regions of the world are closely related (i.e., are probably the same species). Two sets of PKX-specific primers using SSU rDNA gave a strong positive reaction with all kidneys infected with PKX that were tested, including kidneys with only the intraluminal sporogonic form of PKX. These same primers did not amplify rDNA from several fish kidneys infected with Sphaerospora oncorhynchi, S. truttae, S. elegans, or Parvicapsula minibicornis. Sequence comparison of SSU rDNA of PKX with that of S. oncorhynchi showed only 70% similarity across 772 bp (equivalent to 722 bp for PKX). In contrast, intrageneric comparisons of Sphaerospora species among S. oncorhynchi, S. molnari, S. elegans, and putative S. truttae showed them to be essentially identical. This further substantiates evidence that PKX is not S. oncorhynchi. Phylogenetic analysis that used large sections of the SSU rDNA from PKX indicated that PKX is very distinct from other myxosporeans examined thus far and that its roots lie within the Myxozoa before divergence of its two orders, the Multivalvulida and Bivalvulida.