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

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Abstract
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 dif...

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