The molecular phylogeny of trypanosomes: evidence for an early divergence of the Salivaria

Chronic infections with trypanosomes dwelling extracelluarly in the blood and tissues of their hosts are observed in all vertebrate classes. We present here a molecular phylogenetic reconstruction of trypanosome evolution based on nucleotide sequences of small subunit rRNA genes. The evolutionary tree suggests an ancient split into one branch containing all Salivarian trypanosomes and a branch containing all non-Salivarian lineages. The latter branch splits into a clade containing bird, reptilian and Stercorarian trypanosomes infecting mammals and a clade with a branch of fish trypanosomes and a branch of reptilian/amphibian lineages. The branching order of the non-Salivarian trypanosomes supports host-parasite cospeciation scenarios, but also suggests host switches, e.g. between bird and reptilian trypanosomes. The tree is discussed in relation to the modes of adaptation that allow trypanosomes to infect immunocompetent vertebrates. Most importantly, the early divergence of the Salivarian lineages suggests that the presence of a dense proteinaceous surface coat that is subject to antigenic variation is a unique invention of this group of parasites.