Data from: Behavioral response to song and genetic divergence in two subspecies of white-crowned sparrows (Zonotrichia leucophrys)

Divergence in sexual signals may drive reproductive isolation between lineages, but behavioral barriers can weaken in contact zones. Here, we investigate the role of song as a behavioral and genetic barrier in a contact zone between two subspecies of white-crowned sparrows (Zonotrichia leucophrys). We employed a reduced genomic dataset to assess population structure and infer the history underlying divergence, gene flow and hybridization. We also measured divergence in song and tested behavioral responses to song using playback experiments within and outside the contact zone. We found that the subspecies form distinct genetic clusters, and demographic inference supported a model of secondary contact. Song phenotype, particularly length of the first note (a whistle), was a significant predictor of genetic subspecies identity and genetic distance along the hybrid zone, suggesting a close link between song and genetic divergence in this system. Individuals from both parental and admixed localities responded significantly more strongly to their own song than to the other subspecies song, supporting song as a behavioral barrier. Putative parental and admixed individuals were not significantly different in their strength of discrimination between own and other songs; however, individuals from admixed localities tended to discriminate less strongly, and this difference in discrimination strength was explained by song dissimilarity as well as genetic distance. Therefore, we find that song acts as a reproductive isolating mechanism that is potentially weakening in a contact zone between the subspecies. Our findings also support the hypothesis that intra-specific song variation can reduce gene flow between populations.

性信号分化或可驱动支系间生殖隔离，然而在杂交接触带内，行为隔离屏障往往会趋于减弱。本研究聚焦于两种白冠带鹀（Zonotrichia leucophrys）的杂交接触带，探讨鸣唱作为行为与遗传屏障的作用。本研究采用简化基因组数据集，对种群结构进行解析，并推断分化、基因流与杂交事件的演化历史。此外，本研究还量化了鸣唱的分化程度，并通过接触带内外的鸣放回放实验，测试了受试个体对鸣唱的行为反应。研究结果显示，两个亚种各自形成独立的遗传聚类群，种群历史推断结果支持次级接触（secondary contact）模型。鸣唱表型——尤其是首个鸣音（鸣哨声）的时长——可显著预测杂交带内个体的亚种遗传归属与遗传距离，表明该系统中鸣唱与遗传分化存在紧密关联。来自亲本种群分布区与杂交种群分布区的个体，对同种鸣唱的反应强度均显著高于对异种亚种鸣唱的反应，这证实鸣唱可作为行为隔离屏障。推定的亲本个体与杂交个体在同种与异种鸣唱的辨别强度上并无显著差异；但来自杂交分布区的个体，其辨别强度整体偏弱，且这种辨别强度的差异可通过鸣唱相似度与遗传距离加以解释。综上，本研究证实鸣唱可作为生殖隔离机制，而在该亚种间的接触带中，这一机制正趋于弱化。本研究结果同时支持‘种内鸣唱变异可降低种群间基因流’这一假说。