Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada.
Biodiversity Research Centre, University of British Columbia, Vancouver, British Columbia, Canada.
Mol Ecol. 2022 Oct;31(20):5356-5367. doi: 10.1111/mec.16650. Epub 2022 Aug 25.
Isolating barriers that drive speciation are commonly studied in the context of two-species hybrid zones. There is, however, evidence that more complex introgressive relationships are common in nature. Here, we use field observations and genomic analysis, including the sequencing and assembly of a novel reference genome, to study an emergent hybrid zone involving two colliding hybrid zones of three woodpecker species: red-breasted, red-naped, and yellow-bellied sapsuckers (Sphyrapicus ruber, S. nuchalis, and S. varius). Surveys of the area surrounding Prince George, British Columbia, Canada, show that all three species are sympatric, and Genotyping-by-Sequencing identifies hybrids from each species pair and birds with ancestry from all three species. Observations of phenotypes and genotypes of mated pairs provide evidence for assortative mating, though there is some heterospecific pairing. Hybridization is more extensive in this tri-species hybrid zone than in two di-species hybrid zones. However, there is no evidence of a hybrid swarm and admixture is constrained to contact zones, so we classify this region as a tension zone and invoke selection against hybrids as a likely mechanism maintaining species boundaries. Analysis of sapsucker age classes does not show disadvantages in hybrid survival to adulthood, so we speculate the selection upholding the tension zone may involve hybrid fecundity. Gene flow among all sapsuckers in di-species hybrid zones suggests introgression probably occurred before the formation of this tri-species hybrid zone, and might result from bridge hybridization, vagrancies, or other three-species interactions.
在两个物种杂交区的背景下，通常研究驱动物种形成的隔离屏障。然而，有证据表明，更复杂的渐渗关系在自然界中很常见。在这里，我们使用野外观察和基因组分析，包括对一个新的参考基因组进行测序和组装，来研究一个涉及三个啄木鸟物种（红胸、红颈和黄腹啄木鸟）的两个碰撞杂交区的新兴杂交区：红胸、红颈和黄腹啄木鸟（Sphyrapicus ruber、S. nuchalis 和 S. varius）。对加拿大不列颠哥伦比亚省乔治王子城周边地区的调查显示，所有三个物种都是同域的，基因分型测序确定了来自每个种对的杂种和具有三个物种祖先的鸟类。对交配对的表型和基因型的观察为选配提供了证据，尽管存在一些异配。与两个二物种杂交区相比，这个三物种杂交区的杂交更为广泛。然而，没有混合群体的证据，混合仅限于接触区，因此我们将该区域归类为紧张区，并认为对杂种的选择可能是维持物种边界的一种机制。对啄木鸟年龄类别的分析并没有显示杂种在成年后生存的不利，因此我们推测维持紧张区的选择可能涉及杂种的繁殖力。在二物种杂交区的所有啄木鸟之间的基因流动表明，渐渗可能发生在这个三物种杂交区形成之前，可能是由于桥接杂交、漂泊或其他三种物种的相互作用造成的。
