Reconsideration of the phylogenetic position of Frontonia-related Peniculia (Ciliophora, Protozoa) inferred from the small subunit ribosomal RNA gene sequences

For a long time, the class Oligohymenophorea de Puytorac et al., 1974 has been morphologically characterized as a distinct and monophyletic assemblage. However, its relationship of some taxa, especially the Frontonia-related Peniculia have not been much concerned yet using molecular techniques. In this report, the SS rRNA genes for three congeners of Frontonia, namely F. lynni, F. tchibisovae and F didieri, were sequenced and characterized. Phylogenetic trees were constructed by means of Bayesian, neighbor-joining (NJ), least-squares (LS) and maximum parsimony (MP) to assess the inter- and intra-generic relationships of the class Oligohymenophorea. All trees show similar topologies with stable bootstrap support and indicate that: (1) four well known groups, i.e., Peritrichia, Hymenostomatia, Scuticociliatia and Peniculia are distinctly outlined within the class Oligohymenophorea; (2) members of Frontonia are likely more divergent, that is, morphotypes assigned into this genus based on traditional methods might belong to different assemblages, which hence conflicts with the separation of Apofrontonia from the family Frontoniidae, in which the genus Frontonia locates; (3) Apofrontonia always clusters with Paramecium and then groups with the closely related genus Frontonia. In contrast to investigations based on morphology and morphogenesis, the SS rRNA gene sequence determines that the well-outlined genus Frontonia appears to be a paraphyletic assemblage, in which the morphotype Frontonia didieri is more closely related to Apofrontonia than to its congeners.