Hybridization in headwater regions, and the role of rivers as drivers of speciation in Amazonian birds

Many understory birds and other groups form genetically differentiated subspecies or closely related species on opposite sides of major rivers of Amazonia, but are proposed to come into geographic contact in headwater regions where narrower river widths may present less of a dispersal barrier. Whether such forms hybridize in headwater regions is generally unknown, but has important implications to our understanding of the role of rivers as drivers of speciation. We used a dataset of several thousand SNPs to show that seven taxon pairs which differentiate across a major Amazonian river come into geographic contact and hybridize in headwater regions. All taxon pairs possessed hybrids with low numbers of loci in which alleles were inherited from both parental species, suggesting they are backcrossed with parentals, and indicating gene flow between parental populations. Ongoing gene flow challenges rivers as the sole cause of in situ speciation, but is compatible with the view that the wide river courses in the heart of Amazonia may have driven interfluvial divergence during episodes of wet forest retraction away from headwater regions. Taxa as old as 4 Ma in our Amazonian dataset continue to hybridize at contact zones, suggesting reproductive isolation evolves at a slow pace.

亚马逊流域大型河流两岸的林下鸟类及其他类群，普遍演化出了遗传分化的亚种或近缘物种，而学界推测这些类群会在河源区域发生地理接触——此处河道更窄，对个体扩散的阻碍相对较弱。目前学界对这些类群是否会在河源区域发生杂交仍不明确，但该问题对于我们理解河流作为物种形成驱动力的作用具有重要研究价值。本研究利用包含数千个单核苷酸多态性（Single Nucleotide Polymorphisms，SNPs）的数据集，证实了7个在亚马逊大型河流两侧产生分化的类群对，会在河源区域发生地理接触并产生杂交。所有类群对的杂交个体仅携带少量来自双亲物种的等位基因位点，表明这些杂交个体为回交后代，同时证明双亲种群间存在基因交流。持续存在的基因交流对"河流是原地物种形成的唯一驱动力"这一观点提出了挑战，但却与另一假说相符：亚马逊核心区域的宽阔河道，可能在湿林从河源区域退缩的时期，推动了河间区域的类群分化。本研究的亚马逊数据集中，演化时长可达4百万年（Million Years Ago，Ma）的类群仍会在接触带发生杂交，这表明生殖隔离的演化速率较为缓慢。