PHYLOGENY OF TRYPANOSOMES AS INFERRED FROM THE SMALL AND LARGE SUBUNIT RIBOSOMAL-RNAS - IMPLICATIONS FOR THE EVOLUTION OF PARASITISM IN THETRYPANOSOMATID PROTOZOA

Sequences of the small rRNA genes and partial sequences of the large r RNA genes were obtained by PCR amplification from a variety of vertebr ate trypanosomes. The trypanosome species and hosts included Trypanoso ma avium from a bird, T. rotatorium from an amphibian, T. boissoni fro m an elasmobranch, T. triglae from a marine teleost and T. carassii fr om a freshwater teleost. Phylogenetic relationships among these specie s and other represen tatives of the family Trypanosomatidae were infer red using maximum likelihood, maximum parsimony and evolutionary parsi mony. The trypanosomatid tree was rooted using rRNA sequences from two species from the suborder Bodonina. All methods showed that the mamma lian 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 evid ence for co-evolution of trypanosomatids and their hosts, either verte brate or invertebrate. This suggests that evolution of trypanosomatids was accompanied by secondary acquisitions of hosts and habitats.