Ciliate evolution: the ribosomal phylogenies of the tetrahymenine ciliates.

We have assembled and analyzed nucleotide sequences for several different rRNA components from tetrahymenine ciliates. These include previously published and some new 5S and 5.8S rRNAs for a total of 18 species. We also report sequences for some 30 species obtained by primer extension analysis of a region near the 5' end of the 23S rRNAs (region 580). Phylogenetic trees have been constructed for these species, utilizing heuristics (shifting ditypic site analysis) described in a companion paper. The trees based on these sequences are consistent with each other and with those based on longer sequences of the 17S rRNA. They show the tetrahymenines to consist of a number of distinctive clusters of species. The clusters (ribosets) are homogeneous with respect to certain life history characteristics, especially the mode of mating type determination, but are inhomogeneous with respect to some morphological and life history features, such as cyst formation and adaptations to parasitism or carnivory. Using the same molecular data, we also begin to explore the relationships of the tetrahymenines to some other ciliate taxa and to some other protists.