EVOLUTIONARY LINKS BETWEEN TELOMERES AND TRANSPOSABLE ELEMENTS

Transposable elements are abundant in the genomes of higher organisms but are usually thought to affect cells only incidentally, by transpos ing in or near a gene and influencing its expression. Telomeres of Dro sophila chromosomes are maintained by two non-LTR retrotransposons, He T-A and TART These are the first transposable elements with identified roles in chromosome structure. We suggest that these elements may be evolutionarily related to telomerase; in both cases an enzyme extends the end of a chromosome by adding DNA copied from an RNA template. The evolution of transposable elements from chromosomal replication mecha nisms may have occurred multiple times, although in other organisms th e new products have not replaced the endogenous telomerase, as they ha ve in Drosophila. This is somewhat reminiscent of the oncogenes that h ave arisen from cellular genes. Perhaps the viruses that carry oncogen es have also arisen from cellular genetic systems.