| Summary: | <p>Abstract</p> <p>Background</p> <p>Species of <it>Tetrahymena</it> were grouped into three complexes based on morphological and life history traits: the <it>pyriformis</it> complex of microstomatous forms; the <it>patula</it> complex of microstome-macrostome transformers; and the <it>rostrata</it> complex of facultative and obligate histophages. We tested whether these three complexes are paraphyletic using the complete sequence of the small subunit rDNA (SSrDNA).</p> <p>Results</p> <p>In addition to the 16 species of <it>Tetrahymena</it> whose SSrDNA sequences are known, we sequenced the complete SSrDNA from the following histophagous <it>Tetrahymena</it> species; <it>Tetrahymena bergeri, Tetrahymena mobilis, Tetrahymena rostrata,</it> and <it>Tetrahymena setosa</it> as well as the macrostome species <it>Tetrahymena vorax.</it> We also included a ciliate tentatively identified as <it>Lambornella</it> sp., a parasite of the mosquito <it>Aedes</it> sp. We confirmed earlier results using SSrDNA, which showed two distinct clusters of <it>Tetrahymena</it> species: the <it>australis</it> group and <it>borealis</it> group. The genetic distances among <it>Tetrahymena</it> are in general very small. However, all nodes were supported by high bootstrap values. With the exception of <it>T. bergeri</it> and <it>T. corlissi,</it> which are both histophagous and group as sister species, all other histophagous <it>Tetrahymena</it> species are most closely related to a bacterivorous species. Furthermore, <it>Lambornella</it> sp. and <it>T. empidokyrea,</it> both mosquito parasites, are sister species, although there is a considerable genetic distance between them.</p> <p>Conclusions</p> <p>There has been parallel evolution of histophagy in the genus <it>Tetrahymena</it> and the three classical species complexes are paraphyletic. As the genus <it>Lambornella</it> arises within the <it>Tetrahymena</it> clade, it is not likely a defensible one.</p>
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