Are there two classes of VSG gene in Trypanosoma brucei?

Antigenic variation in the African trypanosomes involves the sequential expression of genes coding for different variant surface glycoproteins (VSGs) (reviewed in refs 1-3). When expression of some VSG genes is switched on, a newly duplicated copy of the expressed gene has been observed within the trypanosome genome, which is not found after the gene's expression is switched off again. The duplicated copy has therefore been called an expression-linked copy (ELC). The expression of the gene appears to be strictly coupled to the presence of the ELC. This has led to the hypothesis that the duplicative transposition generating the ELC may itself be responsible for the control of VSG expression. With other VSG genes, expression-linked duplication has not been observed, and expression is clearly not controlled in this way. Data are presented here which demonstrate that either of these observations may be obtained with a single VSG gene, depending on the chance selection of particular clones from antigenically switched populations. Thus, the different observations do not imply the existence of two distinct classes of VSG gene controlled by different mechanisms, but different aspects of processes common to all VSG genes.