Re-analysis of the 18S rRNA gene phylogeny of the ciliate class Colpodea

We critically re-analyzed the 18S rRNA gene phylogeny of the ciliate class Colpodea where four main lineages have been recognized: (1) Bursariomorphida including bryometopids, (2) Platyophryida including sorogenids, (3) Cyrtolophosidida, and (4) Colpodida including bryophryids and grossglockneriids. The Platyophryida branched off first and the Cyrtolophosidida and Colpodida were classified as sister groups. On basis of multiple statistical tests, we unraveled three problematic issues in colpodean phylogenies: the positions of the Bursariomorphida and Platyophryida are unstable and depend on alignment masking; a sister relationship of the Platyophryida and Cyrtolophosidida cannot be excluded by any statistical tree topology test; and clustering of bryophryids and grossglockneriids outside the Colpodida are also statistically valid possibilities. Natural classification of the highly diverse order Colpodida remains puzzling, possibly due to the lack of a phylogenetic signal and morphostasis of the oral ciliature in several Colpoda-like lineages. According to the "Ur-Colpoda" hypothesis, Colpoda represents the stem lineage from which both Colpoda-like and morphologically more derived taxa might have branched off. This evolutionary concept preserves not only information on morphology, ecology, and evolutionary processes of colpodid ciliates, but also aids practicability because the connection to the traditional literature is optimally maintained.