Data from: Male biased sex ratio reduces the fecundity of one of three female morphs in a polymorphic damselfly

Females of the damselfly Ischnura graellsii display 3 color morphs, a male-like androchrome morph and 2 other morphs, infuscans and aurantiaca, which are not male-like. Previous research has suggested that male harassment has a negative effect on female fitness in many different insect species. Studying how male harassment affects fitness of the different female color morphs is key to a better understanding of how these morphs are maintained in natural populations. This study evaluated the response of female morphs of I. graellsii to contrasting sex ratios under controlled laboratory conditions. In our experiments, male abundance, through increased harassment, affected differentially the fecundity of females of the 3 color morphs. A male biased (3:1) sex ratio drastically decreased the average fecundity of infuscans females but had no effect on androchrome and aurantiaca females. Taking into account our results and previous studies that indicate that males prefer infuscans females, we propose a mechanism for the maintenance of this polymorphism. In this scenario, within-generation fluctuations in male abundance produce 2 regimes: One in which male abundance disfavors infuscans females by decreasing their fecundity and other in which a low male abundance results in androchromes that do not mate because of their low appeal to males. By studying a simple population genetics model, we found that the mechanism that we propose may contribute to maintain a stable female-limited polymorphism under a wide range of parameter combinations.

格氏狭翅豆娘（Ischnura graellsii）的雌性个体存在3种体色型：一种为类似雄性的雄同型色型（androchrome），另外两种为非雄同型的infuscans与aurantiaca色型。既往研究表明，在诸多昆虫类群中，雄性骚扰会对雌性的适合度（fitness）产生负面影响。探究雄性骚扰对不同雌性体色型适合度的影响，是阐明此类体色型在自然种群中得以维持的机制的关键所在。本研究在可控实验室条件下，探究了格氏狭翅豆娘雌性体色型对不同性比的响应模式。实验结果显示，雄性密度通过提升骚扰强度，对3种雌性体色型的繁殖力（fecundity）产生了差异化影响。当性比偏向雄性（3:1）时，infuscans雌性的平均繁殖力会大幅下降，而androchrome与aurantiaca雌性的繁殖力则不受影响。结合本研究结果与既往发现（雄性偏好infuscans雌性），我们提出了一套维持该多态性的潜在机制。在此机制框架下，雄性密度在世代内的波动会产生两种情境：其一，高雄性密度通过降低infuscans雌性的繁殖力对其产生不利影响；其二，低雄性密度下，androchrome雌性因对雄性吸引力较弱而难以完成交配。通过构建简单的种群遗传学模型进行分析，我们发现所提出的机制可在广泛的参数组合范围内，助力维持雌性限制性多态（female-limited polymorphism）的稳定性。