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：包括自然选择与差异适合度在内的演化过程，塑造了杂交带内遗传物质的渐渗模式，使得部分基因实现跨种群交流，而另一部分基因则无法完成传递。因此，分子或表型标记的差异化渐渗，可为解析生殖隔离的形成机制提供关键研究视角。本研究以两种潮汐湿地鸟类——盐沼猛雀鹀（Saltmarsh sparrow，Ammodramus caudacutus）与纳尔逊猛雀鹀（Nelson’s sparrow，A. nelsoni）为研究对象，对其杂交带内的遗传变异模式进行了系统表征（样本量n=286），并比较了多种遗传标记与表型性状的渐渗差异。
研究结果：地理与基因组渐变群分析显示，不同标记类型的渐渗模式存在显著异质性。多数标记呈现平缓的渐变群分布，表明渐渗范围超出了此前文献记录的杂交带空间边界。本研究检测到部分陡峭的渐变群分布，说明与潮汐湿地适应相关性状的位点受到强烈正向选择，其中包括与代谢功能（含渗透压调节通路）相关基因区域连锁的分子标记，以及与黑色素沉着相关的表型标记——这一结果支持了深色羽衣（盐沼黑化现象）在潮汐湿地生境中的适应性演化意义。线粒体与性连锁标记的狭窄渐变群分布，同样为霍尔丹法则（Haldane’s rule）提供了实证支持。本研究还发现了偏向A. caudacutus的不对称渐渗模式，该现象可能由两种鸟类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认知视角，同时证实了尽管存在持续的基因流，生态分化在维持物种纯合边界的过程中仍发挥着核心作用。