Identification and phylogenetic relationships within the genus Tetrahymena using the cytochrome c oxidase subunit 1 gene and the small subunit ribosomal RNA gene

Nucleotide sequences of the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene were used as a "DNA barcode" to identify the closely related species of the ciliate genus Tetrahymena and 3 other tetrahymenine species. Except for Tetrahymena furgasoni, Tetrahymena lwoffi, Tetrahymena pyriformis, and Tetrahymena setosa, all isolates could be identified to the species level using 689 bp of the cox1 sequences. Intraspecific isolates of Tetrahymena borealis, T. lwoffi, Tetrahymena patula, and Tetrahymena thermophila could be discriminated and identified by their cox1 sequences as shown by <0.65% intraspecific sequence divegences. However, conspecific individuals of T. pyriformis and Tetrahymena tropicalis showed high "within-species" sequence divergence values of 5.01% and 9.07%, respectively.;Phylogenetic trees constructed using SSUrRNA and cox1 nucleotide and amino acid sequences showed the genus Tetrahymena to be monophyletic as opposed to three morphology-and-ecology-based groupings, which are paraphyletic. The SSUrRNA phylogeny showed the 9 previously established ribosets. The cox1 nucleotide trees revealed 12 well supported groupings, called coxisets, which mostly corresponded to the 9 ribosets.