Fuller Evolutionary Biology Program, Cornell Laboratory of Ornithology, Ithaca, New York 14850.
Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York 14853.
Evolution. 2018 Oct;72(10):2076-2089. doi: 10.1111/evo.13581. Epub 2018 Sep 11.
Natural hybrid zones can be used to dissect the mechanisms driving key evolutionary processes by allowing us to identify genomic regions important for establishing reproductive isolation and that allow for transfer of adaptive variation. We leverage whole-genome data in a system where two bird species, the saltmarsh (Ammospiza caudacuta) and Nelson's (A. nelsoni) sparrow, hybridize despite their relatively high background genetic differentiation and past ecological divergence. Adaptive introgression is plausible in this system because Nelson's sparrows are recent colonists of saltwater marshes, compared to the specialized saltmarsh sparrow that has a longer history of saltmarsh adaptation. Comparisons among whole-genome sequences of 34 individuals from allopatric and sympatric populations show that ongoing gene flow is shaping the genomic landscape, with allopatric populations exhibiting genome-wide F estimates close to double of that observed in sympatry. We characterized patterns of introgression across the genome and identify regions that exhibit biased introgression into hybrids from one parental species. These regions offer compelling candidates for genes related to tidal marsh adaptations suggesting that adaptive introgression may be an important consequence of hybridization. These findings highlight the value of considering the landscapes of both genome-wide introgression and divergence when characterizing the evolutionary forces that drive speciation.
自然杂交区可以通过识别建立生殖隔离和允许适应性变异转移的基因组区域，用于剖析推动关键进化过程的机制。我们利用全基因组数据，研究了两种鸟类——盐沼（Ammospiza caudacuta）和纳尔逊氏（A. nelsoni）麻雀——尽管它们具有相对较高的背景遗传分化和过去的生态分歧，但仍能杂交的系统。在这个系统中，适应性渐渗是合理的，因为与专门适应盐沼的盐沼麻雀相比，纳尔逊氏麻雀是盐水沼泽的最近殖民者。对来自异地种群和同域种群的 34 个个体的全基因组序列进行比较表明，持续的基因流正在塑造基因组景观，异质种群的基因组 F 估计值接近同域种群的两倍。我们描述了整个基因组中渐渗的模式，并确定了表现出偏向于从一个亲本物种向杂种渐渗的区域。这些区域为与潮汐沼泽适应相关的基因提供了引人注目的候选基因，表明适应性渐渗可能是杂交的一个重要后果。这些发现强调了在描述推动物种形成的进化力量时，考虑基因组范围的渐渗和分化景观的价值。
