Data from: Replicate contact zones suggest a limited role of plumage in reproductive isolation among subspecies of the Variable Seedeater (Sporophila corvina)

After establishing secondary contact, recently diverged populations may
remain reproductively isolated or may hybridize to a varying extent
depending on factors such as hybrid fitness and the strength of
assortative mating. Here, we used genomic and phenotypic data from three
independent contact zones between subspecies of the Variable Seedeater
(Sporophila corvina) to examine how coloration and genetic divergence
shape patterns of hybridization. We found that differences in plumage
coloration are likely maintained by divergent selection across contact
zones; however, the degree of plumage differentiation does not match
overall patterns of hybridization. Across two parallel contact zones
between populations with divergent phenotypes (entirely black vs. pied
plumage), populations hybridized extensively across one contact zone but
not the other, suggesting that plumage divergence is not sufficient to
maintain reproductive isolation. Where subspecies hybridized, hybrid zones
were wide and formed by later-generation hybrids, suggesting frequent
reproduction and high survivorship for hybrid individuals. Moreover,
contemporary gene flow has played an important role in shaping patterns of
genetic structure between populations. Replicated contact zones between
hybridizing taxa offer a unique opportunity to explore how different
factors interact to shape patterns of hybridization. Overall, our results
demonstrate that divergence in plumage coloration is important in reducing
gene flow but insufficient in maintaining reproductive isolation in this
clade, and that other factors such as divergence in song and time since
secondary contact may also play an important role in driving patterns of
reduced hybridization and gene flow.

在发生二次接触（secondary contact）后，新近分化的种群既可能保持生殖隔离，也可能依据杂交适合度、选型交配（assortative mating）强度等因素，在不同程度上发生杂交。本研究依托杂色食籽雀（Sporophila corvina）亚种间3个独立接触带的基因组与表型数据，探究羽色分化与遗传分化如何塑造杂交模式。研究结果显示，羽色差异大概率通过各接触带内的歧化选择得以维持；但羽色分化程度与整体杂交模式并不匹配。在两个表型分化显著（通体黑色与斑杂羽型）的平行接触带中，一个接触带内种群发生了广泛杂交，而另一个则未出现此类杂交现象，这表明仅依靠羽色分化不足以维持生殖隔离。在亚种发生杂交的区域，杂交带范围宽阔且由晚世代杂交个体构成，暗示杂交个体具备较高的繁殖频率与存活率。此外，当代基因流在塑造种群间遗传结构模式方面发挥了重要作用。杂交类群间的重复接触带为探究不同因素如何协同作用以塑造杂交模式提供了独特契机。总体而言，本研究结果表明，羽色分化虽可减少基因流，但不足以维持该类群的生殖隔离；鸣唱分化、二次接触后的时长等其他因素，同样在驱动杂交与基因流水平降低的模式中扮演关键角色。