Lemmon Emily Moriarty, Juenger Thomas E
Department of Biological Science Florida State University Tallahassee FL USA.
Department of Integrative Biology University of Texas, Austin Austin TX USA.
Ecol Evol. 2017 Oct 11;7(22):9485-9502. doi: 10.1002/ece3.3443. eCollection 2017 Nov.
Reinforcement contact zones, which are secondary contact zones where species are diverging in reproductive behaviors due to selection against hybridization, represent natural laboratories for studying speciation-in-action. Here, we examined replicate localities across the entire reinforcement contact zone between North American chorus frogs and to investigate geographic variation in hybridization frequencies and to assess whether reinforcement may have contributed to increased genetic divergence within species. Previous work indicated these species have undergone reproductive character displacement (RCD) in male acoustic signals and female preferences due to reinforcement. We also examined acoustic signal variation across the contact zone to assess whether signal characteristics reliably predict hybrid index and to elucidate whether the degree of RCD predicts hybridization rate. Using microsatellites, mitochondrial sequences, and acoustic signal information from >1,000 individuals across >50 localities and ten sympatric focal regions, we demonstrate: (1) hybridization occurs and (2) varies substantially across the geographic range of the contact zone, (3) hybridization is asymmetric and in the direction predicted from observed patterns of asymmetric RCD, (4) in one species, genetic distance is higher between conspecific localities where one or both have been reinforced than between nonreinforced localities, after controlling for geographic distance, (5) acoustic signal characters strongly predict hybrid index, and (6) the degree of RCD does not strongly predict admixture levels. By showing that hybridization occurs in all sympatric localities, this study provides the fifth and final line of evidence that reproductive character displacement is due to reinforcement in the chorus frog contact zone. Furthermore, this work suggests that the dual action of cascade reinforcement and partial geographic isolation is promoting genetic diversification within one of the reinforced species.
强化接触区是物种由于对杂交的选择而在生殖行为上发生分化的次生接触区，是研究正在进行的物种形成的天然实验室。在这里，我们研究了北美合唱蛙整个强化接触区的多个重复地点，以调查杂交频率的地理变异，并评估强化是否可能导致物种内遗传分化增加。先前的研究表明，由于强化作用，这些物种在雄性声学信号和雌性偏好方面经历了生殖特征替代（RCD）。我们还研究了接触区的声学信号变异，以评估信号特征是否能可靠地预测杂交指数，并阐明RCD的程度是否能预测杂交率。利用微卫星、线粒体序列以及来自50多个地点和10个同域焦点区域的1000多个个体的声学信号信息，我们证明：（1）杂交发生，（2）在接触区的地理范围内差异很大，（3）杂交是不对称的，且方向与观察到的不对称RCD模式预测的一致，（4）在一个物种中，在控制地理距离后，一个或两个都经过强化的同域地点之间的遗传距离高于未强化地点之间的遗传距离，（5）声学信号特征能强烈预测杂交指数，（6）RCD的程度并不能强烈预测混合水平。通过表明杂交发生在所有同域地点，本研究提供了第五条也是最后一条证据，证明生殖特征替代是合唱蛙接触区强化的结果。此外，这项工作表明，级联强化和部分地理隔离的双重作用正在促进其中一个强化物种内的遗传多样化。
