Morphological and molecular characterization of a new cyprinid gall bladder-infecting Chloromyxum species, Chloromyxum peleci sp. n. (Myxozoa: Chloromyxidae), from Pelecus cultratus (L.) in Russia

Extensive sampling to get rich data is very important to resolve the current taxonomic problem of Chloromyxum genus and elucidate the nature phylogenetic relationships among congeneric species. During the parasitological survey of cyprinid fish in Gulf of Finland off the coast of St. Petersburg, a new Chloromyxum species, named as Chloromyxum peleci sp. n., was found to infect the gall bladder of sichel, Pelecus cultratus (L.). Here, we provided the morphological, ultrastructural, and molecular features to describe it. Infection was represented by mono- or disporic plasmodia and mature free-floating spores in the gall bladder. Mature spores are typical of freshwater Chloromyxum species, spherical or subspherical in lateral view, measuring 8.8 ± 0.5 μm (7.4–9.5) in length, 7.8 ± 0.3 μm (7.0–8.8) in width, and 7.5 ± 0.4 μm (6.8–8.2) in thickness. Spores have a cog-like appearance in apical view for sutural ridge, and eight to ten widely spaced extrasutural ridges per valve protrude from the spore valve margin. The surface ridge patterns of the new species are similar with those of Chloromyxum auratum and Chloromyxum cristatum, with some branching. Four polar capsules of slightly unequal in two pairs were oval in apical view and pyriform in side view, locating at the anterior end of the spores. Polar filaments coil with four to five turns. The obtained almost full length of 18S ribosomal DNA (rDNA) of Ch. peleci sp. n. does not match any sequences available in GenBank but was most closely related to that of Chloromyxum fluviatile (97.9%). Phylogenetic analysis indicated that Ch. peleci sp. n. clustered in a Chloromyxum subclade infecting the gall bladder of freshwater teleost, with robust nodal support. However, Chloromyxum sensu lato infecting gall bladder of freshwater teleost was again proved to be polyphyletic. The possible evolutionary history of Chloromyxum morphotype of Chloromyxum sensu lato was discussed based on the rDNA-referred phylogeny. This is the second Chloromyxum species from sichel.