Phylogeny and taxonomy of parasites of the Multivalvulida (myxoza:myxosporea) based on comparative DNA sequence analysis

Graduation date: 2004Relationships among myxozoan parasites of the order Multivalvulida were\ud examined through comparative DNA sequence analysis. Members of this group of\ud parasites is known for the damaging effects they have on their fish hosts,\ud especially commercially important species. Most species infect muscle, where\ud they form cysts and many release proteolytic enzymes. Morphological and\ud molecular analyses were conducted on isolates of the widely distributed Kudoa\ud thyrsites to determine if this species was actually an assemblage of several parasite\ud species in a variety of host species. Molecular analyses employed small subunit\ud (SSU) ribosomal DNA sequences and for the first time, myxozoan internal\ud transcribed spacer (ITS) sequences. Genetically distinct populations of K.\ud thyrsites do exist on a broad geographic scale, but it would be inappropriate to\ud consider these groups as different species at this time for the following reasons:\ud there are no clear morphological distinctions that correspond with the observed\ud molecular differences and impacts of the parasite (i.e., soft flesh) are no different in any of the locations. Therefore, although genetic data may suggest that\ud subdivision of K thyrsites is warranted, such subdivision would only generate\ud confusion as DNA sequence analyses would be required for future identifications\ud of these groups. Species boundaries must be defined not only by molecular\ud analyses but by measurable phenotypic differences, as was the case with species\ud similar to K thyrsites; K minithyrsites and Kudoa lateolabracis. At the\ud interspecific level, phylogenetic analyses based on SSU, and for the first time,\ud myxozoan large subunit ribosomal DNA, indicated that a revision of order\ud Multivalvulida taxonomy was warranted. The descriptive generic names\ud Pentacapsula, Hexacapsula and Septemcapsula proved to be artificial groupings\ud of multivalvulid species. Thus, these genera were synonymized with Kudoa. The\ud aim of scientific classification is to organize phena in a manner that not only\ud facilitates communication, but also reflects the shared evolutionary history of these\ud organisms. Clearly, placing members of the Kudoidae into different genera and\ud even families based on a single characteristic of their spores (i.e., valve number) is\ud a poor representation of their close relationship