ANALYSIS OF RIBOSOMAL-RNA GENES SUGGESTS THAT TRYPANOSOMES ARE MONOPHYLETIC

To further investigate the phylogeny of protozoa from the order Kineto plastida we have sequenced the small subunit (SSU) and a portion of th e large subunit (LSU) nuclear rRNA genes. The SSU and LSU sequences we re determined from a lizard trypanosome, Trypanosoma scelopori and a b odonid, Rhynchobodo sp., and the LSU sequences were determined from an insect trypanosomatid, Crithidia oncopelti, and a bodonid, Dimastigel la trypaniformis. Contrary to previous results, in which trypanosomes were found to be paraphyletic, with Trypanosoma brucei representing th e earliest-diverging lineage, we have now found evidence for the monop hyly of trypanosomes. Addition of new taxa which subdivide long branch es (such as that of T. brucei) have helped to identify homoplasies res ponsible for the paraphyletic trees in previous studies. Although the monophyly of the trypanosome clade is supported in the bootstrap analy ses for maximum likelihood at 97% and maximum parsimony at 92%, there is only a small difference in In-likelihood value or tree length betwe en the most optimal monophyletic tree and the best suboptimal paraphyl etic tree. Within the trypanosomatid subtree, the clade of trypanosome s is a sister group to the monophyletic clade of the nontrypanosome ge nera. Different groups of trypanosomes group on the tree according to their mode of transmission. This suggests that the adaptation to inver tebrate vectors plays a more important role in the trypanosome evoluti on than the adaptation to vertebrate hosts.