Putative ITS2 secondary structure model and multi‐gene phylogenies of tetrahymenids (Ciliophora, Hymenostomatia) parasitizing planarians and crayfish worms

Abstract

AbstractTetrahymena comprises over 80 free‐living as well as facultative or obligate histophagous species parasitizing especially invertebrates and fishes. Because of the comparatively high morphological conservativeness of tetrahymenids, their identification is primarily based on the gene coding for cytochrome c oxidase subunit I (COI). In addition to this mitochondrial barcoding marker, we tested here also the species discriminatory power of the highly variable nuclear ITS region, using the secondary structure of the ITS2 molecules and the compensatory base change (CBC) analysis. ITS2 secondary structures were proposed for 25 distinct ribotypes of 13 taxa, including three Tetrahymena species parasitizing planarians and crayfish worms, with the free‐energy minimization approach and the homology modelling. Although the majority of Tetrahymena species pairs can be reliably separated by one to six hemi‐CBCs localized especially in the third and fourth helix of the ITS2 molecules, some ITS region sequences might transcend species boundaries. In other words, intraspecies and interspecies p‐distances of ITS region sequences might overlap. This fact weakens the discriminatory power and the usage of the ITS region in the molecular taxonomy of Tetrahymena. On the other hand, there was always a distinct barcoding gap at about 4% sequence divergence in the case of the COI gene. Despite the slightly lower delimitation power of the ITS region, this molecular marker helped to recognize multiple previous misidentifications that were subsequently verified by comparing COI sequences.