EVOLUTION OF 16S-LIKE RIBOSOMAL-RNA GENES IN THE CILIOPHORAN TAXA LITOSTOMATEA AND PHYLLOPHARYNGEA

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 Trithigmos toma steini). Six of the eight ciliate classes are now represented in the 16S-like rRNA data base. Phylogenetic reconstructions identify Ble pharisma americanum as the first ciliate branch but the heterotrichs B . americanum and Metopus palaeformis do not appear to be closely relat ed. Later branching ciliate lineages are represented by the litostomes , spirotrichs and a heterogeneous group that includes colpodid, oligoh ymenophoran, and phyllopharyngean ciliates. The relatively simple cyto stomal 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 complex ity of their oral structures can be interpreted as a secondary adaptat ion - possibly caused by a shift towards macrophagous feeding behavior .