Phylogenetic relationships within the class Spirotrichea (Ciliophora) inferred from small subunit rRNA gene sequences

The small subunit rDNAs of five species belonging to the Euplotidae and eig ht species of the Oxytrichidae were sequenced to obtain a more detailed pic ture of the phylogenetic relationships within the Spirotrichea (Ciliophora) . Various tree reconstruction algorhythms yielded nearly identical topologi es. All Euplotidae were separated from the other Spirotrichea by a deep spl it. Further, a large genetic distance between the marine genus Moneuplotes and the freshwater species of Euplotoides was found. Differences between th e methods used occurred only within the Oxytrichidae. Whereas the monophyly of the Stylonychinae was supported in all trees, the monophyly of the Oxyt richinae was not. However, the molecular data support the morphological and ontogenetic evidence that the pattern of 18 frontal-ventral-transversal ci rri evolved in the stemline of the Oxytrichidae and was modified several ti mes independently. Our results are also in agreement with taxonomic revisio ns: the separation of both Sterkiella nova from Oxytricha and Tetmemena pus tulata from Stylonychia. (C) 2001 Academic Press.