The traditional taxonomy of the genus Chloromyxum Mingazzini, 1890 has been intensively challenged to be paraphyletic by recent ribosomal DNA (rDNA)-based phylogenetic analysis. Undersampling to get rich sequence data to infer more scientific phylogenetic relationships makes scientists conservatively assign all non-marine elasmobranch-infecting species as Chloromyxum sensu lato. Although complex ridge pattern on the spore surface observed by scanning electron microscopy was thought to be critical for the identification of Chloromyxum species, insufficient data also prevent this ultrastructural data to be a valid taxonomic feature for this genus. It is especial for Chloromyxum species to be reported in China. Molecular and ultrastructural characteristics are yet available for all 22 Chloromyxum species recorded in China. During the investigation of the diversity of coelozoic fish myxosporeans, Chloromyxum ellipticum Li & Nie, 1973 was found to highly infect the gall bladder of Ctenopharyngodon idellus Valenciennes, 1844 in Poyang Lake watershed of Jiangxi province, Eastern China. Here, we redescribed it by the currently recommended holistic approach of combining morphological, ultrastructural, and molecular characteristics. Mature spores were found floating free in the gall bladder, but no plasmodium observed. Spores are typical freshwater teleost-infecting Chloromyxum species, spherical or subspherical in lateral view, measuring 7.7 ± 0.08 μm (6.9-9.1) in length, 6.3 ± 0.09 μm (5.6-7.6) in width, and 5.8 ± 0.20 μm (5.2-6.3) in thickness. Four pyriform polar capsules, located at the anterior end of the spores, were equal in size, 3.3 ± 0.06 μm (2.2-4.1) long and 2.1 ± 0.03 μm (1.7-2.5) wide. Polar filaments coiled with four to five turns. Two equal spore valves are symmetrical, with 10-16 surface extrasutural ridges per valve, aligned along the longitudinal axis. The obtained partial 18S rDNA of C. ellipticum did not match any sequences available in GenBank. Phylogenetic analysis showed that C. ellipticum clustered firstly with Chloromyxum legeri with robust nodal support and grouped then with urinary system of freshwater teleost-infecting Chloromyxum clade, rather than other gall bladder of freshwater teleost-infecting clade.
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.
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