Rapid evolution of genes coding for variant surface glycoproteins in trypanosomes.

We have used cloned DNA complementary to the messenger RNAs (mRNAs) for different variant surface glycoproteins (VSGs) of Trypanosoma brucei, stock 427, to study the degree of conservation of the corresponding nuclear genes in related trypanosome stocks. Conservation of restriction endonuclease cleavage sites in and around these genes were assessed by hybridization of the complementary DNA (cDNA) probes to nuclear DNA blots of these stocks. One of the genes (117) was found essentially unaltered in 11 out of 12 stocks. A second gene (118) was absent in five stocks. In the seven stocks that contained it, four forms of this 118 gene could be distinguished that differ by loss/gain of several restriction sites. A third gene (221) was only present in T. brucei 427 and in none of 11 other stocks. We conclude that a sub-set of the genes for the variant antigens evolves at a very high rate and we favour the hypothesis that this is due to local hypermutagenesis.