Data from: Females discriminate against heterospecific sperm in a natural hybrid zone

When hybridization is maladaptive, species-specific mate preferences are selectively favored, but low mate availability may constrain species-assortative pairing. Females paired to heterospecifics may then benefit by copulating with multiple males and subsequently favoring sperm of conspecifics. Whether such mechanisms for biasing paternity towards conspecifics act as important reproductive barriers in socially monogamous vertebrate species remains to be determined. We use a combination of long-term breeding records from a natural hybrid zone between collared and pied flycatchers (Ficedulla albicollis and F. hypoleuca), and an in vitro experiment comparing conspecific and heterospecific sperm performance in female reproductive tract fluid, to evaluate the potential significance of female cryptic choice. We show that the females most at risk of hybridizing (pied flycatchers) frequently copulate with multiple males and are able to inhibit heterospecific sperm performance. The negative effect on heterospecific sperm performance was strongest in pied flycatcher females that were most likely to have been previously exposed to collared flycatcher sperm. We thus demonstrate that a reproductive barrier acts after copulation but before fertilization in a socially monogamous vertebrate. While the evolutionary history of this barrier is unknown, our results imply that there is opportunity for it to be accentuated via a reinforcement-like process.

当杂交存在适应度代价时，物种特异性的配偶偏好会受到自然选择的青睐，但配偶可获得性不足可能会限制物种选配配对的发生。与异种（heterospecific）雄性配对的雌性，或许可通过与多个雄性交配，并后续优先利用同种（conspecific）精子的方式获益。这类将父本偏向同种个体的机制，是否会成为社会一夫一妻制脊椎动物类群中重要的生殖屏障，仍有待明确。
本研究结合领姬鹟（Ficedulla albicollis）与斑姬鹟（F. hypoleuca）自然杂交带的长期繁殖记录，以及一项在雌性生殖道液中比较同种与异种精子性能的体外实验，以评估雌性隐蔽配偶选择的潜在重要性。研究结果表明，杂交风险最高的斑姬鹟雌性会频繁与多个雄性交配，且能够抑制异种精子的活性。这种对异种精子性能的抑制效应，在此前最可能接触过领姬鹟精子的斑姬鹟雌性中表现得最为显著。
由此可见，在社会一夫一妻制的脊椎动物中，存在一类发生在交配后、受精前的生殖屏障。尽管该屏障的演化历史尚未明确，但我们的研究结果表明，其存在为该屏障通过类强化选择过程得到强化提供了潜在可能。