Low levels of transposable element activity in Drosophila mauritiana: causes and consequences

Transposable elements (TEs) are major drivers of genomic and phenotypic evolution, yet many questions about their biology remain poorly understood. Here, we compare TE abundance between populations of the two sister species D. mauritiana und D. simulans and relate it to the more distantly related D. melanogaster. The low population frequency of most TE insertions in D. melanogaster and D. simulans has been a key feature of several models of TE evolution. In D. mauritiana, however, the majority of TE insertions are fixed (66%). We attribute this to a lower transposition activity of up to 47 TE families in D. mauritiana, rather than stronger purifying selection. Only three families, including the extensively studied Mariner, may have a higher activity in D. mauritiana. This remarkable difference in TE activity between two recently diverged Drosophila species ({approx} 250,000 years), also supports the hypothesis that TE copy numbers in Drosophila may not reflect a stable equilibrium where the rate of TE gains equals the rate of TE losses by negative selection. We propose that the transposition rate heterogeneity results from the contrasting ecology of the two species: the extent of vertical extinction of TE families and horizontal acquisition of active TE copies may be very different between the colonizing D. simulans and the island endemic D. mauritiana. Our findings provide novel insights in the evolution of TEs in Drosophila and suggest that the ecology of the host species could be a major, yet underappreciated, factor governing the evolutionary dynamics of TEs.