P -elements strengthen reproductive isolation within the Drosophila simulans species complex

Determining mechanisms that underlie reproductive isolation is key to understanding how species boundaries are maintained in nature. Transposable elements (TEs) are ubiquitous across eukaryotic genomes. However, the role of TEs in modulating the strength of reproductive isolation between species is poorly understood. Several species of have been found to harbor P-elements (PEs), yet only is known to be polymorphic for their presence in wild populations. PEs can cause reproductive isolation between PE-containing (P) and PE-lacking (M) lineages of the same species. However, it is unclear whether they also contribute to the magnitude of reproductive isolation between species. Here, we use the species complex to assess whether differences in PE status between and its sister species, which do not harbor PEs, contribute to multiple barriers to gene flow between species. We show that crosses involving a P father and an M mother from a sister species exhibit lower F1 female fecundity than crosses involving an M father and an M sister-species mother. Our results suggest that the presence of PEs in a species can strengthen isolation from its sister species, providing evidence that transposable elements can play a role in reproductive isolation and facilitate the process of speciation.