Long-term evolution of therootransposable element copy number in mutation accumulation lines ofDrosophila melanogaster

Abstract

SummaryThe rate of insertion of transposable elements (TEs) is a fundamental parameter to understand both their dynamics and role in the evolution of the eukaryotic genome. Nonetheless, direct estimates of insertion rates are scarce because transposition is in general a rare phenomenon. A great deal of our previous work on transposition was based on a set of long-term mutation accumulation (MA) lines ofDrosophila melanogasterstarted in 1987 (Oviedo lines), whereroowas found highly active, with a rate of insertion of 7×10−4insertions per element and generation, as compared with other 15 TE families that presented transposition rates around 10−5. Here, we study the evolution of therootransposition rate, byin situhybridization, after 60–75 additional generations of MA in two subsets of the Oviedo lines, O and O′, which had achieved average numbers ofrooinsertions of 77 and 84, respectively. In the O lines, insertions accumulated at a rate that remained constant (7×10−4insertions per element and generation); however, the subset of lines O′ showed a lower accumulation rate of 4×10−4insertions per element per generation, suggesting a regulation of transposition that depends on the number of elements. However, one of the O′ lines reached a number of 103 insertions, departing from the group mean by 4·6sd, and showing that it escapes regulation. Hence, ‘de novo’ mutations affecting the regulation of transposition are relatively common. These results are discussed in relation to the possible mechanisms of containment of TEs.