Abstract
The phylogeny of the genus Paratetrahymena was studied based on molecular, morphological and morphogenetic evidence of a poorly known species, P. wassi (The Virginia Journal of Science, 14, 1963, 126). Phylogenetic trees constructed with sequences of small subunit rRNA (SSU-rRNA) demonstrate that P. wassi, together with another morphotype of this genus and Cardiostomatella vermiforme, always form a distinct sister group to the typical scuticociliates and are clearly isolated from the Hymenostomatia and Peritrichia clades. Its stomatogenesis, however, is like that of hymenostomes in that the parental oral structure is completely inherited by the proter and the new oral anlagen of the opisthe derives from the right postoral kinety, thus supporting a close relationship with the Hymenostomatia. The only conclusions that can be reached from molecular, morphological and morphogenetic information are that (1) Paratetrahymena seems to belong to the order Loxocephalida and (2) clarifying the classification, phylogenetic position and composition of the Loxocephalida still depends on obtaining a much more inclusive set of all kinds of data. As a further contribution, a new diagnosis and morphological redescription of Paratetrahymena and P. wassi are provided based on new data. It is interesting that molecular evidence also suggests that the morphologically unique Urocentrum, which has been classified as a peniculid for a long time, is closely related to a putative loxocephalid, Dexiotrichides pangi, and that both might occupy a phylogenetic position distant from the taxa to which they are assigned at present.