Figure 2.

Potential steps in the evolution of Drosophila telomeres. Several steps were probably involved in the evolution of the HeT-A and TART system of telomere maintenance. Two steps could have occurred sequentially or in parallel: these are the loss of sequence specificity for the capping function of telomeres, and the recombinational maintenance of telomeres after loss of telomerase. After loss of telomerase and acquisition of an ALT mechanism, the telomere repeats, if still present, may be replaced by other tandem repeats. Finally, retrotransposable elements that may already be present take over the end-maintenance function.