REDESCRIPTION AND MOLECULAR PHYLOGENETIC ANALYSIS ON MYXOBOLUS ACUTUS WU AND CHEN, 1987 (MYXOZOA: MYXOBOLIDAE)

Myxosporeans are obiligate endoparasites belonging to the phylum Cnidaria that includes mainly free-living species, such as sea anemones, hydras or jellyfish. Myxosporeans infecting wild and farmed fishes represent an important pathogenic group with a worldwide distribution. Myxosporidiosis can cause severe ecological and economic effects, which draw extensive attention to many people. Although myxosporeans were discovered almost 200 years ago, their classification was debated for a long time, especially about their higher taxonomic placement. Until recently, many discoveries sealed the demise of Myxozoa as a phylum and supported its inclusion in Cnidaria. Up to now, there are some 2600 described myxosporeans species with only about 23% of them have molecular sequence data. Therefore, there are still a lot of molecular information of known species to be supplemented and perfected. The traditional classification and identification of these species were based on their spore morphology. However, many taxonomists are aware of the limitations of the morphology-only classification because of the simplified body structure and the plasticity of some myxospores. Nowadays, almost all studies about taxonomy of myxosporeans are conducted using a combination of morphological and molecular data. Myxobolus acutus Wu and Chen, 1987 was first reported from gills and kidneys of Carassius auratus Linnaeus collected from Hubei, China, in the year 1983—1984. Wu and Chen named the species and gave a full description of the morphology of the parasite but without molecular data. This study redescribed the species infected the gills and gallbladders of C. auratus from Jialing River of Chongqing using the 18S rDNA and ITS1 rDNA sequences and its morphological and molecular data. The mature myxospores of M. acutus were pyriform with slightly narrow anterior and blunt posterior in valvular view. They were wide spindle-shaped in sutural view. The spores were (13.6±0.9) μm (11.4—15.3) μm in length, (10.2±0.9) μm (7.5—12.8) μm in width and (7.6±0.6) μm (6.9—8.3) μm in thickness. Two polar capsules with pores adjoined were unequal in the anterior of the spore. The larger polar capsules were pyriform with (6.2±0.4) μm (5.1—7.5) µm long and (3.8±0.4) μm (2.8—4.7) μm wide and 5—8 turns of polar filaments; the smaller ones were pyriform with (2.7±0.4) μm (1.7—3.7) μm long and (1.4±0.2) μm (0.9—1.9) μm wide and 2—3 turns of polar filaments. The 18S rDNA sequence similarity, genetic distance and variation site of the 7 strains of M. acutus were 100%, 0.000 and 0, respectively. ITS1 sequence analysis from three strains of M. acutus reflected genetic diversity and the different genetic resources of the two genotypes. Phylogenetic analysis based on 18S rDNA revealed that M. acutus and Thelohanellus sinensis had the closest relationship and they were clustered into a clade sister to the clade composing of M. musseliusae, M. tsangwuensis and M. basilamellaris. The analysis also indicates that the original parasitic site of M. acutus might be gill while gallbladder should be the later adaptive site of M. acutus.