Data from: Low fertility of wild hybrid male flycatchers despite recent divergence

Postzygotic isolation may be important for maintaining species boundaries, particularly when premating barriers are incomplete. Little is known about the course of events leading from minor environmental mismatches affecting hybrid fitness to severe genetic incompatibilities causing sterility or inviability. We investigated whether reduced reproductive success of hybrid males was caused by suboptimal sperm traits or by more severe genetic incompatibilities in a hybrid zone of pied (Ficedula hypoleuca) and collared flycatchers (F. albicollis) on the island of Öland, Sweden. About 4 per cent hybridization is observed in this population and all female hybrids are sterile. We found no sperm in the ejaculates of most sampled hybrid males, and sperm with abnormal morphology in two hybrids. Furthermore, none of the hybrids sired any offspring because of high levels of hatching failure and extra-pair paternity in their nests. These results from a natural hybrid zone suggest that the spermatogenesis of hybrid males may become disrupted despite little genetic divergence between the parental species.合子后生殖隔离（postzygotic isolation）对于维持物种边界或具有重要意义，尤其当交配前屏障（premating barriers）不完备时。目前学界对由影响杂种适合度（hybrid fitness）的轻微环境差异，逐步演变为导致不育（sterility）或无生活力（inviability）的严重遗传不相容性（genetic incompatibilities）的过程，仍知之甚少。本研究以瑞典厄兰岛（Öland）上的斑鹟（Ficedula hypoleuca）与领鹟（F. albicollis）的杂交带（hybrid zone）为研究对象，旨在探究杂种雄性繁殖成功率降低的诱因，究竟是精子性状（sperm traits）欠佳，还是更为严重的遗传不相容性所致。该种群中约有4%的杂交事件发生，且所有雌性杂种均表现为不育。研究发现，多数采样的杂种雄性的精液（ejaculates）中未检测到精子，另有两只杂种的精子存在形态异常（abnormal morphology）。此外，由于其巢内出现高水平的孵化失败（hatching failure）与巢外父权（extra-pair paternity）现象，所有杂种均未成功繁育后代。这项基于自然杂交带的研究结果表明，尽管亲本物种间的遗传分化（genetic divergence）程度较低，但杂种雄性的精子发生（spermatogenesis）过程或已遭到破坏。