Contact zones provide important insights into the evolutionary processes that underlie lineage divergence and speciation. Here, we use a contact zone to ascertain speciation potential in the red-eyed treefrog (Agalychnis callidryas), a brightly coloured and polymorphic frog that exhibits unusually high levels of intraspecific variation. Populations of A. callidryas differ in a number of traits, several of which are known sexual signals that mediate premating reproductive isolation in allopatric populations. Along the Caribbean coast of Costa Rica, a ~100 km contact zone, situated between two phenotypically and genetically divergent parent populations, contains multiple colour pattern phenotypes and late-generation hybrids. This contact zone provides the opportunity to examine processes that are important in the earliest stages of lineage divergence. We performed analyses of colour pattern variation in five contact zone sites and six parental sites and found complex, continuous colour variation along the contact zone. We found discordance between the geographic distribution of colour pattern and previously described genomic population structure. We then used a parental site and contact zone site to measure assortative mating and directional selection from naturally-occurring amplectant mating pairs. We found assortative mating in a parental population, but no assortative mating in the contact zone. Furthermore, we uncovered evidence of directional preference towards the adjacent parental phenotype in the contact zone population, but no directional preference in the parent population. Combined, these data provide insights into potential dynamics at the contact zone borders and indicate that incipient speciation between parent populations will be slowed.

中文翻译：

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跨种内接触区的性信号演化和选型交配模式。

接触区为了解谱系分歧和物种形成的进化过程提供了重要的见解。在这里，我们使用接触区来确定红眼树蛙（Agalychnis callidryas）的物种形成潜力，这是一种颜色鲜艳的多态性青蛙，表现出异常高水平的种内变异。A. callidryas 种群在许多特征上存在差异，其中一些特征是已知的性信号，可介导异域种群中交配前的生殖隔离。沿着哥斯达黎加的加勒比海岸，一个约 100 公里的接触区位于两个表型和遗传上不同的亲本群体之间，包含多种颜色模式表型和晚代杂交种。该接触区提供了检查谱系分歧早期阶段重要过程的机会。我们对五个接触区位点和六个亲本位点的颜色图案变化进行了分析，发现沿着接触区的复杂、连续的颜色变化。我们发现颜色模式的地理分布与之前描述的基因组群体结构之间存在不一致。然后，我们使用亲本位点和接触区位点来测量自然发生的半接触交配对的选型交配和方向选择。我们在亲代群体中发现了选型交配，但在接触区没有选型交配。此外，我们发现了接触区群体中对相邻亲本表型的方向偏好的证据，但亲代群体中没有方向偏好。综合起来，这些数据提供了对接触区边界潜在动态的深入了解，并表明亲代种群之间的初期物种形成将会减慢。