Molecular characterization and phylogenetic analysis of Paratrichodina africana Kazubski and El-Tantawy, 1986 based on 18S rRNA gene data with the evolutionary hypothesis of trichodinids.

Abstract

In this study, we provided the morphological data and the first 18S rRNA gene data of Paratrichodina africana Kazubski and El-Tantawy, 1986, isolated from hybrids of Oreochromis niloticus × Oreochromis mossambicus in Chongqing, China. Morphologically, P. africana is mainly characterized by the triangular blade and prominent anterior projection. The present population is consistent with the original populations in the overall appearance of the adhesive disc, and falls within the morphometry range of the original descriptions. Phylogenetically, P. africana was clustered into one large clade with Trichodinella and Tripartiella species, which was nested within Trichodina ones with strong support. By combining morphological and molecular data, our results revealed that the validity of the genus Paratrichodina was doubtful, and suggested that the three genera Trichodinella, Tripartiella, and Paratrichodina should be incorporated into one independent genus. In addition, we provided morphological and molecular data of additional eight trichodinids, and further performed the phylogenetic analysis and traced the evolution history of trichodinids' five morphological and bionomical characters for the first time by taking advantage of the current GenBank data. According to the present results, one evolutionary hypothesis of trichodinids was proposed as follows. The most recent common ancestor of trichodinids inhabiting the freshwater environment as a symbiont of vertebrates should evolve from the ancestor with a long-spiral adoral ciliary turn. The first differentiated Trichodina species should be parasitic on one vertebrate distributed in the freshwater environment. During their evolution, some trichodinids expanded to the marine environment, and some switched to invertebrates in the freshwater environment. The denticle of some freshwater Trichodina species became narrower, and the adoral ciliary spiral turn got shorter, forming the ancestor-oid organism with a short-spiral adoral ciliary turn. Then, those Trichodinella, Tripartiella, and Paratrichodina species might evolve from those ancestor-oid organisms with short-spiral adoral ciliary turn.