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