Data from: Flexible mate choice may contribute to ecotype assortative mating in pumpkinseed sunfish (Lepomis gibbosus)

Gene flow is expected to limit adaptive divergence but the ecological and behavioural factors that govern gene flow are still poorly understood, particularly at the earliest stages of population divergence. Reduced gene flow through mate choice (sexual isolation) can evolve even under conditions of subtle population divergence if intermediate phenotypes have reduced fitness. We indirectly tested the hypothesis that mate choice has evolved between coexisting littoral and pelagic ecotypes of polyphenic pumpkinseed sunfish (Lepomis gibbosus) that have diverged in morphology and resource use and where intermediate phenotypes have reduced performance. We assessed the ecotype of nesting males and females using stable isotope estimates of diet and a divergent male morphological trait, oral jaw width. We found positive assortative mating between ecotypes in a common spawning habitat along exposed lake shorelines, but contrary to expectations, assortative mating was variably expressed between two sampling years. Although the factors that influence variable assortative mating remain unclear, our results are consistent with mate choice being expressed by ecotypes. Despite being variably expressed, mate choice will reduce gene flow between ecotypes and could contribute to further adaptive divergence depending on its frequency and strength in the population. Our findings add to a growing body of evidence indicating mate choice behaviour can be a plastic trait, an idea that should be more explicitly considered in empirical studies of mate choice as well as conceptual frameworks of mate choice evolution and adaptive divergence.

基因流（gene flow）本应限制适应性分化（adaptive divergence），但调控基因流的生态与行为机制仍不甚明晰，尤其是在种群分化的最早阶段。若中间表型的适合度降低，即便种群分化程度微弱，通过配偶选择（mate choice）实现的基因流降低（即性隔离（sexual isolation））也可能演化。我们间接检验了这一假说：在形态与资源利用上已产生分化、且中间表型性能下降的多态性南瓜籽太阳鱼（Lepomis gibbosus）的共存沿岸型与远洋型生态型之间，配偶选择已经演化。我们通过基于稳定同位素（stable isotope）的饮食估算，以及雄性的分化形态特征——口腔颌骨宽度（oral jaw width），对筑巢雌雄个体的生态型进行了判定。我们发现，在暴露湖岸线的共同产卵栖息地中，生态型间存在正选型交配（positive assortative mating）；但与预期相悖的是，两年采样间的正选型交配表现存在差异。尽管影响可变正选型交配的因素仍不明确，但我们的结果与生态型存在配偶选择的结论一致。尽管配偶选择的表现存在波动，但其仍会降低生态型间的基因流，并可能根据其在种群中的发生频率与强度，进一步推动适应性分化。我们的研究结果补充了日益增多的证据，表明配偶选择行为可作为一种可塑性性状（plastic trait）；这一观点应在配偶选择的实证研究，以及配偶选择演化与适应性分化的概念框架中得到更明确的考量。