The surface structure of trypanosomes in relation to their molecular phylogeny

Molecular phylogenetic analysis using genes coding for ribosomal RNA and pr oteins suggests that trypanosomes are monophyletic. Salivarian trypanosomes showing antigenic variation of the variant surface glycoprotein (VSG) dive rged from non-Salivarian trypanosomes some 200-300 million years ago. Repre sentatives of the non-Salivarian group, the mammalian parasite, Trypanosoma cruzi, and the freshwater fish trypanosome, T. carassii, are characterised by surfaces dominated by carbohydrate-rich mucin-like glycoproteins, which are not subject to antigenetic variation. It is suggested that this latter surface structure is typical for non-Salivarian trypanosomes as well as me mbers of the other Kinetoplastid suborder, the Bodonina. This would imply t hat at some point in time in the evolution of the Salivaria the highly abun dant and comparatively poorly immunogenetic mucin-like molecules must have been replaced for equally abundant but highly immunogenic VSG-like molecule s. While the selective advantage for such a unique transition is difficult to imagine, the subsequent diversification of VSG genes/molecules may have been comparatively straightforward because even the most limited form of an tigenic variation would have extended the duration of infection in the vert ebrate and thus would have increased the chance for transfer to the vector. (C) 2001 Australian Society for Parasitology Inc. Published by Elsevier Sc ience Ltd. All rights reserved.