Taxonomy of Kudoa species (Myxosporea), using a small-subunit ribosomal DNA sequence

Abstract

Myxosporeans in the genus Kudoa infect the flesh of many marine fishes and often cause unsightly lesions and softening of the flesh texture. We are particularly interested in K. thyrsites because it is associated with soft flesh in the Atlantic salmon (Salmo solar), an important commercial species in Canada. Sequences of the small-subunit (SSU) rDNA (about 1600 base pairs) were obtained from K. miniauriculata, K. amanuensis, and K. poniformis. We aligned these sequences with one obtained from coho salmon (Oncorhynchus kisutch) and designed "Kudoa general" primers (KUD1f and KUD2r). These primers, in combination with other general primers, were then used to obtain the SSU rDNA sequence of K. thyrsites from two host species, Atlantic salmon and tubesnout (Aulorhynchus flavidus), from British Columbia, Canada. Sequence comparisons of these isolates indicated that Kudoa species cluster by geographic location rather than by morphology of spores. The three species from the eastern Pacific were approximately 97% identical, whereas K. amamiensis (from Japan) was about 91% identical with these species. Sequence comparisons of K. thyrsites from Atlantic salmon and tubesnout revealed a difference of only 0.07% between these isolates. Comparison of SSU rDNA sequences from the four Kudoa species and Henneguya salminicolo analyzed in this study with those from other available myxosporean genera (Myxidium and Myxoholus) showed that taxonomic divisions at the order and suborder levels were consistent with classical views of the taxonomy of the Myxosporea. Using specific regions of the SSU rDNA, we also developed a sensitive and specific polymerase chain reaction test for detection of K. thyrsites.