Data from: Human disturbance causes the formation of a hybrid swarm between two naturally sympatric fish species

Most evidence for hybrid swarm formation stemming from anthropogenic habitat disturbance comes from the breakdown of reproductive isolation between incipient species, or introgression between allopatric species following secondary contact. Human impacts on hybridization between divergent species that naturally occur in sympatry has received considerably less attention. Theory predicts that reinforcement should act to preserve reproductive isolation under such circumstances, potentially making reproductive barriers resistant to human habitat alteration. Using 15 microsatellites we examined hybridization between sympatric populations of alewife (Alosa pseudoharengus) and blueback herring (A. aestivalis) to test whether the frequency of hybridization and pattern of introgression have been impacted by the construction of a dam that isolated formerly anadromous populations of both species in a landlocked freshwater reservoir. The frequency of hybridization and pattern of introgression differed markedly between anadromous and landlocked populations. The range-wide frequency of hybridization among anadromous populations was generally 0-8%, whereas all landlocked individuals were hybrids. Although neutral introgression was observed among anadromous hybrids, directional introgression leading to increased prevalence of alewife genotypes was detected among landlocked hybrids. We demonstrate that habitat alteration can lead to hybrid swarm formation between divergent species that naturally occur sympatrically, and provide empirical evidence that reinforcement does not always sustain reproductive isolation under such circumstances.

人为栖息地干扰（anthropogenic habitat disturbance）引发的杂交群（hybrid swarm）形成现象，其相关证据大多来源于雏形物种（incipient species）间生殖隔离（reproductive isolation）的瓦解，或是异域物种（allopatric species）经二次接触（secondary contact）后发生的基因渐渗（introgression）。然而，针对自然同域分布（sympatry）的分化物种（divergent species）间杂交受人类活动影响的研究，却鲜有关注。理论预测，在此类情形下，生殖强化（reinforcement）作用应会维系物种间的生殖隔离，使得生殖屏障能够抵御人类栖息地改造的影响。本研究利用15个微卫星标记（microsatellites），对自然同域分布的灰西鲱（alewife, Alosa pseudoharengus）与蓝背西鲱（blueback herring, A. aestivalis）种群间的杂交现象开展检测，以验证水坝修建是否对二者的杂交频率与基因渐渗模式产生了影响——该水坝将两种鱼原本的溯河洄游种群（anadromous populations）隔离至内陆淡水水库（landlocked freshwater reservoir）中。溯河洄游种群与内陆种群间的杂交频率及基因渐渗模式存在显著差异。全域范围内溯河洄游种群的杂交频率普遍为0%~8%，而所有内陆个体均为杂交个体。尽管在溯河洄游杂交个体中观察到中性基因渐渗（neutral introgression）现象，但内陆杂交个体中却检测到定向基因渐渗（directional introgression），该过程导致灰西鲱基因型的占比显著升高。本研究证实，栖息地改造可引发自然同域分布的分化物种间形成杂交群，并为生殖强化并非总能在此类情形下维系生殖隔离提供了实证依据。