Data from: 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 competitive setting, indicating no difficulty of sperm from Vancouver males to fertilize Colorado eggs after double matings. While there were subtle differences in how P2 changed over time, suggesting that Vancouver males’ sperm are somewhat less competitive in a first-male role within Colorado females, these effects did not translate into differences in overall P2. Fertilisation success can thus differ dramatically between competitive and noncompetitive conditions, perhaps because the males that mate second produce higher quality ejaculates in response to sperm competition. We suggest that unlike in more divergent species comparisons, where sperm competition typically increases reproductive isolation, ejaculate tailoring can reduce the potential for PMPZ isolation when recently diverged populations interbreed.

交配后合子前（Postmating but Prezygotic, PMPZ）互作，如今日益被学界认可为生殖隔离演化进程中一类具有潜在重要性的早期屏障。近期一项研究报道了同物种种群间的一则潜在案例：蒙大拿果蝇（Drosophila montana）不同种群间的单次交配实验中，受精成功率存在显著差异——源于其中一种群（温哥华种群）的精子无法穿透另一种群（科罗拉多种群）的卵子。鉴于蒙大拿果蝇的自然交配体系为多配制（雌性可快速多次交配），我们开展了同种群间所有可能杂交组合的双交配实验，以探究射精间的竞争效应是否会对该PMPZ生殖隔离产生影响。我们测定了无选择交配实验中的交配前隔离水平、雌性繁殖力、单次及双交配后的受精成功率、卵至成虫存活率，以及第二任父本父权成功率（P2）。令人意外的是，在竞争交配情境下，我们未检测到两个种群间存在PMPZ生殖隔离，这表明双交配后温哥华种群雄性的精子可顺利使科罗拉多种群的卵子完成受精。尽管P2随时间推移存在细微变化，提示温哥华雄性的精子在科罗拉多雌性体内充当第一任父本时竞争力略有不足，但这类效应并未转化为整体P2的显著差异。由此可见，竞争与非竞争情境下的受精成功率可能存在显著差异，这或许是因为第二任交配的雄性会因精子竞争压力而产出质量更高的射精产物。我们提出，与物种种间分化程度较高的对比研究不同——此类研究中精子竞争通常会强化生殖隔离——当新近分化的种群发生杂交时，射精适配可降低PMPZ生殖隔离的潜在发生概率。