Molecular characterization and phylogenetic analyses of Tripartiella macrosoma Basson and Van As, 1987 and Tripartiella obtusa Ergens and Lom, 1970 based on 18S rRNA gene sequence data

In the present study, we provided the first 18S rRNA gene sequence data of two Tripartiella species, Tripartiella macrosoma Basson and Van As, 1987 and Tripartiella obtusa Ergens and Lom, 1970, which were isolated from Tachysurus fulvidraco (Richardson, 1846) and Hemibarbus maculatus Bleeker, 1871 in Chongqing, China, respectively. Morphologically, both species fall within the morphometry range of the original descriptions and are very similar to the original populations in the overall appearance of the adhesive disc. Tripartiella macrosoma can be easily distinguished from the other Tripartiella species by possessing the denticle with a long strip and conspicuously inclined backward blade and a robust and short ray. Tripartiella obtusa is mainly characterized by a broad blade and a relatively long ray. Phylogenetically, T. macrosoma clustered with Trichodinella myakkae (Mueller, 1937) Raabe, 1950 and further with Trichodinella sp., which was sister to a group that includes four populations of Trichodinella epizootica (Raabe, 1950) Šrámek-Hušek, 1953; finally, they formed a small clade with T. obtusa. This result suggested that T. macrosoma had a closer relationship with Trichodinella spp. than with T. obtusa and T. obtusa diverged earlier than T. macrosoma and Trichodinella spp. By combining morphological and molecular data, the polyphyletic characteristics of Tripartiella and Trichodinella were further analyzed, and the results revealed that the validity of the genus Tripartiella is doubtful.