Postmating-prezygotic isolation between two allopatric populations of Drosophila montana: fertilisation success differs under sperm competition

Postmating but prezygotic (PMPZ) interactions are increasingly recognized as a potentially important early-stage barrier in the evolution of reproductive isolation. A recent study described a potential example between populations of the same species: single matings between Drosophila montana populations resulted in differential fertilisation success because of the inability of sperm from one population (Vancouver) to penetrate the eggs of the other population (Colorado). As the natural mating system of D. montana is polyandrous (females remate rapidly), we set up double matings of all possible crosses between the same populations to test whether competitive effects between ejaculates influence this PMPZ isolation. We measured premating isolation in no-choice tests, female fecundity, fertility and egg-to-adult viability after single and double matings as well as second-male paternity success (P2). Surprisingly, we found no PMPZ reproductive isolation between the two populations under a c...