Systematics and Molecular Phylogeny of the Ciliate Genus Pseudokeronopsis (Ciliophora, Hypotrichia)

A still challenging study on ciliate systematics is to clarify the taxonomic and phylogenetic confusions of the fascinating pigmented Pseudokeronopsis species because of their high interspecific similarities. We evaluated the identities and internal relationships within Pseudokeronopsis based on morphological descriptions in combination with the 18S and ITS-5.8S rDNA genetic distances, ITS2 secondary structures and phylogenetic analyses. Results showed that (1) Pseudokeronopsis pararubra, a species ever synonymized as Pseudokeronopsis carnea, is a valid species; (2) most Chinese populations identified as Pseudokeronopsis rubra represent a new species namely Pseudokeronopsis songi sp. nov.; (3) within Pseudokeronopsis, P. carnea branched early, while P. songi sp. nov., P. flava, and P. erythrina showed close relationships and formed a sister clade with P. pararubra; (4) compared to 18S and ITS2 rDNA, ITS-5.8S rDNA is more suitable for separating Pseudokeronopsis species, with pairwise distances of 0-0.0441 at the intraspecific level and 0.0635-0.1150 at the interspecific level; and (5) the core structure of Pseudokeronopsis ITS2 includes three helices, with helix III being the longest and showing an identical pattern in conspecific populations and small differences among species. Based on the evaluation, we clarify all misidentified and dubious 18S and ITS-5.8S rDNA sequences of Pseudokeronopsis in GenBank.