THE MOLECULAR PHYLOGENY OF TRYPANOSOMES - EVIDENCE FOR AN EARLY DIVERGENCE OF THE SALIVARIA

Chronic infections with trypanosomes dwelling extracelluarly in the bl ood and tissues of their hosts an 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 contai ning all Salivarian trypanosomes and a branch containing all non-Saliv arian 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/amphibia n lineages. The branching order of the non-Salivarian trypanosomes sup ports host-parasite cospeciation scenarios, but also suggests host swi tches, e.g. between bird and reptilian trypanosomes. The tree is discu ssed in relation to the modes of adaptation that allow trypanosomes to infect immunocompetent vertebrates. Most importantly, the early diver gence 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. (C) 1998 Francqui Found ation. Published Elsevier Science B.V. All rights reserved.