PHYLOGENY OF TRICHOMONADS INFERRED FROM SMALL-SUBUNIT RIBOSOMAL-RNA SEQUENCES

Small subunit (16S-like) ribosomal RNA sequences were obtained from re presentatives of all four families constituting the order Trichomonadi da. Comparative sequence analysis revealed that the Trichomonadida are a monophyletic lineage and a deep branch of the eukaryotic tree. Rela tive to other early divergent eukaryotic assemblages the branching pat tern within the Trichomonadida is very shallow. This pattern suggests the Trichomonadida radiated recently, perhaps in conjunction with thei r animal hosts. From a morphological perspective the Devescovinidae an d Calonymphidae are considered more derived than the Monocercomonadida e and Trichomonadidae. Molecular trees inferred by distance, parsimony and likelihood techniques consistently show the Devescovinidae and Ca lonymphidae are the earliest diverging lineages within the Trichomonad ida, however bootstrap values do not strongly support a particular bra nching order. In an analysis of all known 16S-like ribosomal RNA seque nces, the Trichomonadida share most recent common ancestry with uniden tified protists from the hindgut of the termite Reticulitermes flavipe s. The position of two putative free-living trichomonads in the tree i s indicative of derivation from symbionts rather than direct descent f rom some free-living ancestral trichomonad.