PHYLOGENY OF TRICHOMONADS BASED ON PARTIAL SEQUENCES OF LARGE SUBUNITRIBOSOMAL-RNA AND ON CLADISTIC-ANALYSIS OF MORPHOLOGICAL DATA

Several domains of large subunit rRNA from nine trichomonad species ha ve been sequenced. Molecular phylogenies obtained with parsimony and d istance methods demonstrate the trichomonads are a monophyletic group which branches very early in the eukaryotic tree. The topology of the trees is in general agreement with traditional views on evolutionary a nd systematic relationships of trichomonads. A clear dichotomy is note d between the subfamily Trichomonadinae and the subfamily Tritrichomon adinae. In the latter subfamily, a second division separates the ''Tri trichomonas muris-type'' species from the ''Tritrichomonas augusta-typ e'' ones. Previous evolutionary schemes in which the Monocercomonadida e were regarded as the most ''primitive'' and the Trichomonadidae as m ore ''evolved'' are not in agreement with our molecular data, The emer gence of Monocercomonas and Hypotrichomonas at the base of the Tritric homonas lineage suggests a secondary loss of some cytoskeletal structu res, the costa and undulating membrane in these genera. This is corrob orated by the early branching position of Trichomitus. which possesses a costa and an undulating membrane and has usually been placed among the Trichomonadidae on the basis of cytological characters. A cladisti c analysis was applied to the available morphological characters in or der to produce a hierarchical grouping of the taxa reflecting their mo rphological diversity.