Principal Investigator Mary-Lou Pardue
Transposable elements have significantly added to and shaped eukaryotic genomes. Unfortunately, difficulties in assembling long sequences of highly repetitive DNA have largely precluded analysis of the organization and possible roles of transposable elements in heterochromatic regions such as centromeres and telomeres. Recently published sequence of a BAC (Bacterial Artificial Chromosome) containing sequence from the centromere of the D. melanogaster Y chromosome allowed us to compare sequences in centromeric and telomeric heterochromatin. That BAC contains an array of telomeric HeT-A elements which apparently transposed from a telomere into the Y centromere more than 13 Myr ago. It has remained there as the genus evolved and is now found in all species in the D. melanogaster group of species.
We found that the HeT-A arrays have been maintained quite differently in those two heterochromatic regions (telomere and centromere), resulting in sequence organizations that reflect different roles in the two chromosomal environments. The telomere array has grown only by transposition of new elements to the chromosome end; whereas the centromeric array has grown by repeated amplification of segments of the original telomere array. In the telomere, many elements have undergone variable 5’-truncation by gradual erosion and irregular deletion of the chromosome end; however a significant fraction remain complete and capable of further retrotransposition. In contrast, each element in the centromere region has lost 40% or more of its sequence and the loss occurred by internal, rather than terminal, deletions. No centromere element retains a significant part of its original coding region. Thus the centromeric array has been restructured to resemble the highly repetitive satellite sequences typical of centromeres in multicellular organisms; meanwhile, over a similar or longer time period, the telomere array has maintained its ability to provide retrotransposons competent to extend telomere ends.