The phylogeny of Myxosporea (Myxozoa) based on small subunit ribosomal RNA gene analysis

The phylogeny of the Myxosporea was studied using the small-subunit ribosomal RNA gene sequences. Maximum parsimony and Bayesian inference were used to determine myxosporean phylogenetic relationships. The analysis included 120 myxosporean sequences retrieved from GenBank and 21 newly obtained sequences of myxosporeans representing nine genera. Members of the genera Palliatus and Auerbachia were sequenced for the first time. The phylogenetic analysis supported a split of myxosporeans into two main lineages separating most of freshwater species from marine ones as described by previous authors. In addition to the two main lineages, a third lineage consisting of three species was found (Sphaerospora truttae, Sphaerospora elegans and Leptotheca ranae) and additional exceptions to the marine/freshwater myxosporean split were recognised (Sphaeromyxa hellandi, Sphaeromyxa longa and Myxidium coryphaenoideum). All three myxosporean lineages were characterised by specific lengths of SSU rDNA sequences. The lineage of marine myxosporeans split into five well-defined clades. They consisted of species with a similar site of infection and spore morphology and were referred as the Parvicapsula clade, the Enteromyxum clade, the Ceratomyxa clade, the marine Myxidium clade and the Kudoa clade, respectively. The inner topology of the freshwater clade was more complex but the trend to branch according to site of infection was observed in this clade as well. Due to the number of sequences available, a histozoic (Myxobolus clade) predominated. Interestingly, five morphologically different species infecting urinary bladder clustered within the histozoic (Myxobolus) clade. The phylogenetic trees derived from this study differ in a number of respects from the current taxonomy of the myxosporeans, which suggests that several currently utilised characters may be homoplasious or that reliance on a single gene tree may not adequately reflect the phylogeny of the group.