Data from: Strong selection on male plumage in a hybrid zone between a hybrid bird species and one of its parents

Homoploid hybrid speciation (HHS) requires reproductive barriers between hybrid and parent species, despite incomplete reproductive isolation (RI) between the parents. Novel secondary sexual trait values in hybrids may cause prezygotic isolation from both parents, whereas signals inherited by the hybrid from one parent species may cause prezygotic isolation with the other. Here we investigate whether differences in male plumage function as a premating barrier between the hybrid Italian sparrow and one of its parent species, the house sparrow, in a narrow Alpine hybrid zone. Italian sparrow male plumage is a composite mosaic of the parental traits, with its head plumage most similar to its other parent, the Spanish sparrow. We use geographical cline analysis to examine selection on three plumage traits, 75 nuclear single nucleotide polymorphisms (SNPs) and hybrid indices based on these SNPs. Several SNPs showed evidence of restricted introgression in the Alps, supporting earlier findings. Crown colour exhibited the narrowest plumage cline, representing a 37% (range 4–65%) drop in fitness. The cline was too narrow to be due to neutral introgression. Only crown colour was significantly bimodal in the hybrid zone. Bimodality may be due to RI or a major QTL, although fitness estimates suggest that selection contributes to the pattern. We discuss the implications with respect to HHS and the species status of the Italian sparrow.同倍体杂交物种形成（Homoploid hybrid speciation, HHS）要求杂交种与亲本物种之间形成生殖屏障，尽管亲本间存在不完全的生殖隔离（reproductive isolation, RI）。杂交个体中新出现的第二性征特征值，可能使其与双亲均产生合子前隔离；而杂交个体从某一亲本物种继承的信号，则可能使其与另一亲本产生合子前隔离。本研究旨在探究：在狭窄的阿尔卑斯杂交带中，雄性羽色差异是否可作为意大利麻雀（该研究中的杂交物种）与其亲本之一家麻雀之间的交配前生殖屏障。意大利麻雀的雄性羽色是亲本性状的复合镶嵌体，其头部羽色与另一亲本西班牙麻雀最为相似。我们采用地理渐变群分析方法，针对3项羽色性状、75个核单核苷酸多态性（single nucleotide polymorphisms, SNPs）以及基于这些SNPs构建的杂交指数开展选择作用分析。多项SNPs呈现出在阿尔卑斯区域受限的基因渐渗证据，这佐证了此前的研究结论。在所有羽色性状中，头顶羽色的渐变群宽度最窄，对应的适合度下降幅度达37%（区间为4%至65%）。该渐变群的宽度过窄，无法以中性渐渗进行解释。在该杂交带中，仅头顶羽色呈现出显著的双峰分布特征。这一双峰分布可能源于生殖隔离或某一主效数量性状基因座（Quantitative Trait Locus, QTL），尽管适合度估算结果显示选择作用参与塑造了该分布模式。最后我们就同倍体杂交物种形成以及意大利麻雀的物种地位问题，讨论了本研究的相关启示。