Phylogenetic relationships between prostome and colpodean ciliates tested by small subunit rRNA sequences.

We analyzed the phylogenetic relationships among the four ciliate classes, Prostomatea, Colpodea, Nassophorea, and Litostomatea, by small subunit ribosomal RNA gene sequence comparison to test several phylogenetic hypotheses that had been proposed for these classes, based on ultrastructural features. The complete coding regions of the prostomes, Prorodon teres and Coleps hirtus, and of the colpodean, Bursaria truncatella, were determined and compared to the database and the literature. The prostomes and colpodeans are strongly supported as monophyletic groups in all analyses. A sister-group relationship between prostomes and colpodeans is strongly supported by parsimony analyses and moderately supported by distance matrix analyses. There was no support for a close relationship between colpodeans and nassophoreans or between colpodeans and litostomes. In all analyses, these four classes branch late in the ciliate tree. This finding further supports the new view of ciliate phylogeny that so-called ancestral taxa, such as the "primitive" prostomes, are in fact derived and that so-called "highly evolved" groups, such as the heterotrichs, branch off very early in the ciliate tree.