Coevolution of the Telomeric Retrotransposons Across Drosophila Species

As in other eukaryotes, telomeres in Drosophila melanogaster are composed of long arrays of repeated DNA sequences. Remarkably, in D. melanogaster these repeats are produced, not by telomerase, but by successive transpositions of two telomere-specific retrotransposons, HeT-A and TART. These are the only transposable elements known to be completely dedicated to a role in chromosomes, a finding that provides an opportunity for investigating questions about the evolution of telomeres, telomerase, and the transposable elements themselves. Recent studies of D. yakuba revealed the presence of HeT-A elements with precisely the same unusual characteristics as HeT-Amel although they had only 55% nucleotide sequence identity. We now report that the second element, TART, is also a telomere component in D. yakuba; thus, these two elements have been evolving together since before the separation of the melanogaster and yakuba species complexes. Like HeT-Ayak, TART yak is undergoing concerted sequence evolution, yet they retain the unusual features TART mel shares with HeT-Amel. There are at least two subfamilies of TART yak with significantly different sequence and expression. Surprisingly, one subfamily of TART yak has >95% sequence identity with a subfamily of TART mel and shows similar transcription patterns. As in D. melanogaster, other retrotransposons are excluded from the D. yakuba terminal arrays studied to date.