Phylogeny of trypanosomes as inferred from the small and large subunit rRNAs: implications for the evolution of parasitism in the trypanosomatid protozoa.

Sequences of the small rRNA genes and partial sequences of the large rRNA genes were obtained by PCR amplification from a variety of vertebrate trypanosomes. The trypanosome species and hosts included Trypanosoma avium from a bird, T. rotatorium from an amphibian, T. boissoni from an elasmobranch, T. triglae from a marine teleost and T. carassii from a freshwater teleost. Phylogenetic relationships among these species and other representatives of the family Trypanosomatidae were inferred using maximum likelihood, maximum parsimony and evolutionary parsimony. The trypanosomatid tree was rooted using rRNA sequences from two species from the suborder Bodonina. All methods showed that the mammalian parasite, Trypanosoma brucei, constitutes the earliest divergent branch. The remaining trypanosomes formed a monophyletic group. Within this group, the bird trypanosome was grouped with T. cruzi, while the elasmobranch trypanosome and the two fish trypanosome species formed a group with an affinity to T. rotatorium. Our results provide no evidence for co-evolution of trypanosomatids and their hosts, either vertebrate or invertebrate. This suggests that evolution of trypanosomatids was accompanied by secondary acquisitions of hosts and habitats.