Phylogenetic position of the trichomonad parasite of turkeys, Histomonas meleagridis (Smith) Tyzzer, inferred from small subunit rRNA sequence

The phylogenetic position of the trichomonad, Histomonas meleagridis was de termined by analysis of small subunit rRNAs. Molecular trees including all identified parabasalid sequences available in data bases were inferred by d istance, parsimony, and likelihood methods. All reveal a close relationship between H. meleagridis, and Dientamoeba fragilis. Moreover, small subunit rRNAs of both amoeboid species have a reduced G + C content and increased c hain length relative to other parabasalids. Finally, the rRNA genes from H. meleagridis and D. fragilis share a recent common ancestor with Tritrichom onas foetus. which exhibits a more developed cytoskeleton. This indicates t hat Histomonas and Dientamoeba secondarily lost most of the typical trichom onad cytoskeletal structures and hence, do not represent primitive morpholo gies. A global phylogeny of parabasalids revealed significant discrepancies with morphology-based classifications, such as the polyphyly of most of th e parabasalid families and classes included in our study.