Data from: Differential introgression and the maintenance of species boundaries in an advanced generation avian hybrid zone

Background: Evolutionary processes, including selection and differential fitness, shape the introgression of genetic material across a hybrid zone, resulting in the exchange of some genes but not others. Differential introgression of molecular or phenotypic markers can thus provide insight into factors contributing to reproductive isolation. We characterized patterns of genetic variation across a hybrid zone between two tidal marsh birds, Saltmarsh (Ammodramus caudacutus) and Nelson’s (A. nelsoni) sparrows (n = 286), and compared patterns of introgression among multiple genetic markers and phenotypic traits. Results: Geographic and genomic cline analyses revealed variable patterns of introgression among marker types. Most markers exhibited gradual clines and indicated that introgression exceeds the spatial extent of the previously documented hybrid zone. We found steeper clines, indicating strong selection for loci associated with traits related to tidal marsh adaptations, including for a marker linked to a gene region associated with metabolic functions, including an osmotic regulatory pathway, as well as for a marker related to melanin-based pigmentation, supporting an adaptive role of darker plumage (salt marsh melanism) in tidal marshes. Narrow clines at mitochondrial and sex-linked markers also offer support for Haldane’s rule. We detected patterns of asymmetrical introgression toward A. caudacutus, which may be driven by differences in mating strategy or differences in population density between the two species. Conclusions: Our findings offer insight into the dynamics of a hybrid zone traversing a unique environmental gradient and provide evidence for a role of ecological divergence in the maintenance of pure species boundaries despite ongoing gene flow.

研究背景：包括选择与适合度差异在内的进化过程，塑造了杂交带内遗传物质的渐渗模式，使得部分基因而非全部基因发生交流。分子或表型标记的差异性渐渗，可为解析促进生殖隔离的相关因素提供线索。本研究对两种潮汐湿地鸟类——盐沼带鹀（英文名Saltmarsh sparrow，学名Ammodramus caudacutus）与尼尔森带鹀（英文名Nelson’s sparrow，学名A. nelsoni）的杂交带（样本量n=286）的遗传变异模式进行了表征，并比较了多种遗传标记与表型性状间的渐渗差异。

研究结果：地理梯度与基因组梯度分析显示，不同标记类型的渐渗模式存在显著差异。多数标记呈现平缓的梯度分布，表明渐渗范围超出了此前文献记载的杂交带空间跨度。本研究同时发现了更为陡峭的遗传梯度，这意味着与潮汐湿地适应性性状相关的基因座受到了强烈选择——其中包括一个与代谢功能（含渗透压调节通路）相关基因区域连锁的标记，以及一个与黑色素沉着相关的标记，该结果支持了深色羽衣（盐沼黑化现象，salt marsh melanism）在潮汐湿地生境中的适应性作用。线粒体与性连锁标记的狭窄梯度分布也为霍尔丹法则（Haldane’s rule）提供了实证支持。本研究还检测到了偏向A. caudacutus的不对称渐渗模式，该模式可能由两个物种的交配策略差异或种群密度差异所驱动。

研究结论：本研究结果为解析跨越独特环境梯度的杂交带动态提供了新的研究视角，同时证实了尽管存在持续的基因流，生态分化在维持纯种物种边界的过程中发挥了关键作用。