Abstract
Complete small subunit (16S-like rRNA) gene sequences were determined for two litostome (Homalozoon vermiculare and Loxophyllum utriculariae) and two phyllopharyngean ciliates (Discophrya collini and Trithigmostoma steini). Six of the eight ciliate classes are now represented in the 16S-like rRNA data base. Phylogenetic reconstructions identify Blepharisma americanum as the first ciliate branch but the heterotrichs B. americanum and Metopus palaeformis do not appear to be closely related. Later branching ciliate lineages are represented by the litostomes, spirotrichs and a heterogeneous group that includes colpodid, oligohymenophoran, and phyllopharyngean ciliates. The relatively simple cytostomal organization of litostomes and phyllopharyngeans should not be regarded as primitive states. Using the rRNA phylogenies as framework to understand the evolution of non-molecular features, the low complexity of their oral structures can be interpreted as a secondary adaptation — possibly caused by a shift towards macro-phagous feeding behavior.
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