Sex and evolution in trypanosomes

Trypanosoma brucei is still the only kinetoplastid known to undergo genetic exchange, but it seems unreasonable to suppose that it evolved this proces s all by itself. The position of T. brucei on a molecular phylogenetic tree constructed from 18S ribosomal RNA gene sequences offers no clues to the l ikely existence of genetic exchange in trypanosome species other than the S alivaria, because this group of trypanosomes appears to have diverged from the rest a very long time ago. Antigenic variation is one characteristic sh ared by the Salivaria, which has been particularly well-studied in T. bruce i. The large proportion of the genome devoted to variant antigen genes and related sequences in T. brucei, suggests a possible role for genetic exchan ge in enhancing the diversity of the repertoire. Alternatively, genetic exc hange may counter potential excessive double-strand DNA damage brought abou t by the DNA rearrangements associated with antigenic variation. The remark able biparental inheritance of organelle DNA ( = kinetoplast DNA) in T. bru cei is without precedent in other eukaryotes. The result of genetic exchang e is to enhance the heterogeneity of the kinetoplast DNA minicircles. (C) 2 001 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved.