Department of Biological Sciences, University of Alabama, Tuscaloosa, Alabama.
Department of Environmental Health Science and Institute of Bioinformatics, University of Georgia, Athens, Georgia.
Mol Ecol. 2019 Feb;28(4):761-771. doi: 10.1111/mec.14983. Epub 2019 Feb 6.
Speciation is the result of an accumulation of reproductive barriers between populations, but pinpointing the factors involved is often difficult. However, hybrid zones can form when these barriers are not complete, especially when lineages come into contact in intermediate or modified habitats. We examine a hybrid zone between two closely related riverine turtle species, Sternotherus depressus and S. peltifer, and use dual-digest RAD sequencing to understand how this hybrid zone formed and elucidate genomic patterns of reproductive isolation. First, the geographical extent and timing of formation of the hybrid zone is established to provide context for understanding the role of extrinsic and intrinsic reproductive isolating mechanisms in this system. The strength of selection on taxon-specific contributions to maintenance of the hybrid zone is then inferred using a Bayesian genomic cline model. These analyses identify a role for selection inhibiting introgression in some genomic regions at one end of the hybrid zone and promoting introgression in many loci at the other. When selective pressures necessary to generate outliers to the genomic cline are considered with the geographical and temporal context of this hybrid zone, we conclude that habitat-specific selection probably limits introgression from S. depressus to S. peltifer in the direction of river flow. However, selection is mediating rapid, unidirectional introgression from S. peltifer to S. depressus, which is probably facilitated by anthropogenic habitat alteration. These findings indicate a potentially imminent threat of population-level genomic extinction for an already imperiled species due to ongoing human-caused habitat alteration.
物种形成是种群间生殖隔离积累的结果，但确定相关因素通常很困难。然而，当这些障碍不完全存在时，尤其是当谱系在中间或改良的栖息地中接触时，杂交区就会形成。我们研究了两种密切相关的河流龟类，Sternotherus depressus 和 S. peltifer 之间的杂交区，并使用双酶切 RAD 测序来了解这个杂交区是如何形成的，并阐明生殖隔离的基因组模式。首先，建立了杂交区的地理范围和形成时间，为理解该系统中外在和内在生殖隔离机制的作用提供了背景。然后，使用贝叶斯基因组渐变模型推断分类群特异性贡献维持杂交区的选择强度。这些分析确定了选择在杂交区的一端抑制某些基因组区域的渐渗，而在另一端促进许多基因座的渐渗的作用。当考虑到这个杂交区的地理和时间背景下产生基因组渐变异常值所需的选择压力时，我们得出结论，特定栖息地的选择可能限制了从 S. depressus 到 S. peltifer 的渐渗，方向是河流流动的方向。然而，选择介导了从 S. peltifer 到 S. depressus 的快速、单向渐渗，这可能是人为改变栖息地造成的。这些发现表明，由于人类造成的栖息地不断改变，一个已经受到威胁的物种可能面临种群水平基因组灭绝的迫在眉睫威胁。
