New insights into the phylogeny of trichomonads inferred from small subunit rRNA sequences

Small subunit ribosomal DNA sequences were obtained by polymerase chain rea ction from four trichomonad species: a frog endosymbiont Trichomitus batrac horum, an intestinal endosymbiont of a squamate reptile, Hypotrichomonas ac osta and two free-living isolates, Monotrichomonas carabina and Monotrichom onas sp. Molecular trees inferred by distance, parsimony and likelihood tec hniques identify three well-resolved clusters within the trichomonads, howe ver bootstrap values do not strongly support a particular branching order f or these lineages, The first cluster includes the Devescovinidae and the Ca lonymphidae. The second clade unites Trichomitus batrachorum and Hypotricho monas acosta. The third cluster embraces all known free-living genera, incl uding Monotrichomonas, and various members of the Trichomonadinae subfamily such as Trichomonas vaginalis, and Pentatrichomonoides scroa, Neither Mono cercomonadidae nor the Trichomonadidae as envisaged are monophyletic. Most of the monocercomonads, which possess a rudimentary cytoskeleton, were like ly descendants of more complex forms. The study also suggests that the genu s Trichomitus is currently polyphyletic, partly explaining the discordant p ositions of this genus in previous molecular analyses.