Scordato Elizabeth S C, Wilkins Matthew R, Semenov Georgy, Rubtsov Alexander S, Kane Nolan C, Safran Rebecca J
Department of Ecology and Evolutionary Biology, The University of Colorado, Boulder, CO, USA.
School of Biological Sciences, University of Nebraska-Lincoln, Lincoln, NE, USA.
Mol Ecol. 2017 Oct;26(20):5676-5691. doi: 10.1111/mec.14276. Epub 2017 Sep 5.
Hybrid zones are geographic regions where isolating barriers between divergent populations are challenged by admixture. Identifying factors that facilitate or inhibit hybridization in sympatry can illuminate the processes that maintain those reproductive barriers. We analysed patterns of hybridization and phenotypic variation across two newly discovered hybrid zones between three subspecies of barn swallow (Hirundo rustica). These subspecies differ in ventral coloration and wing length, traits that are targets of sexual and natural selection, respectively, and are associated with genome-wide differentiation in allopatry. We tested the hypothesis that the degree of divergence in these traits is associated with the extent of hybridization in secondary contact. We applied measures of population structure based on >23,000 SNPs to confirm that named subspecies correspond to distinct genomic clusters, and assessed coincidence between geographic clines for ancestry and phenotype. Although gene flow was ongoing across both hybrid zones and pairwise F between subspecies was extremely low, we found striking differences in the extent of hybridization. In the more phenotypically differentiated subspecies pair, clines for ancestry, wing length and ventral coloration were steep and coincident, suggestive of strong isolation and, potentially, selection associated with phenotype. In the less phenotypically differentiated pair, gene flow and phenotypic variation occurred over a wide geographic span, indicative of weaker isolation. Traits associated with genome-wide differentiation in allopatry may thus also contribute to isolation in sympatry. We discuss potentially important additional roles for evolutionary history and ecology in shaping variation in the extent hybridization between closely related pairs of subspecies.
杂交带是地理区域，在这些区域中，不同种群之间的隔离屏障受到混合的挑战。确定促进或抑制同域杂交的因素可以阐明维持这些生殖屏障的过程。我们分析了家燕（Hirundo rustica）三个亚种之间两个新发现的杂交带中的杂交模式和表型变异。这些亚种在腹部颜色和翅长方面存在差异，这两个性状分别是性选择和自然选择的目标，并且在异域分布中与全基因组分化相关。我们检验了这样一个假设：这些性状的分化程度与二次接触中的杂交程度相关。我们应用基于超过23000个单核苷酸多态性（SNP）的种群结构测量方法，以确认命名的亚种对应于不同的基因组簇，并评估祖先和表型的地理渐变之间的一致性。尽管两个杂交带都存在持续的基因流动，且亚种之间的成对F值极低，但我们发现杂交程度存在显著差异。在表型分化较大的亚种对中，祖先、翅长和腹部颜色的渐变陡峭且一致，这表明存在强烈的隔离，并且可能存在与表型相关的选择。在表型分化较小的亚种对中，基因流动和表型变异发生在广泛的地理范围内，这表明隔离较弱。因此，在异域分布中与全基因组分化相关的性状也可能有助于同域分布中的隔离。我们讨论了进化历史和生态学在塑造密切相关的亚种对之间杂交程度变异方面可能具有的重要额外作用。
