Affiliation of Cochlosoma to trichomonads confirmed by phylogenetic analysis of the small-subunit rRNA gene and a new family concept of the order Trichomonadida

The protozoan genus Cochlosoma includes parasitic intestinal flagellates of birds and mammals of uncertain taxonomic classification. The presence of an adhesive disc, superficially similar to that of Giardia, led to a proposal that Cochlosoma should be classified as diplomonads. Careful morphological and ultrastructural observations, however, revealed conspicuous homologies to trichomonads. We addressed the question of classification and phylogenetic affiliation of Cochlosoma using the methods of molecular phylogenetics. Analyses based on the 16S rRNA gene sequence of the species Cochlosoma anatis very robustly placed Cochlosoma in the clade of the parabasalid subfamilies Trichomonadinae, Trichomitopsiinae and Pentatrichomonoidinae of the order Trichomonadida (bootstraps >94 %). The data did not provide robust support for any particular position of Cochlosoma within this clade because the sequence suffered from mutational saturation and produced a long branch. The most probable sister taxon of Cochlosoma is the genus Pentatrichomonas, because their relationship was supported specifically by the slowest-mutating, least-saturated positions as determined using the method slow-fast. Classification of the order Trichomonadida was revised to accommodate knowledge about its phylogeny - the family Cochlosomatidae and subfamilies Trichomitopsiinae and Pentatrichomonoidinae were abandoned, Trichomonadidae was amended and new families Tritrichomonadidae (formerly a subfamily) and Trichomitidae were proposed.