Long-term evolution of the roo transposable element copy number in mutation accumulation lines of Drosophila melanogaster.

The 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 of Drosophila melanogaster started in 1987 (Oviedo lines), where roo was found highly active, with a rate of insertion of 7×10-4 insertions per element and generation, as compared with other 15 TE families that presented transposition rates around 10-5. Here, we study the evolution of the roo transposition rate, by in situ hybridization, after 60-75 additional generations of MA in two subsets of the Oviedo lines, O and O', which had achieved average numbers of roo insertions of 77 and 84, respectively. In the O lines, insertions accumulated at a rate that remained constant (7×10-4 insertions per element and generation); however, the subset of lines O' showed a lower accumulation rate of 4×10-4 insertions 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 4·6 sd, 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.