Data from: Disruptive sexual selection on body size in the polyphenic black scavenger fly Sepsis thoracica

Sexual selection has two main components, female preference and male-male competition, which can lead males to adopt alternative reproductive tactics to optimize their reproductive success. Two traits that significantly influence reproductive success are body size and coloration, as they can facilitate access to females through male contests or as female attractors. We investigated whether, and if so which mechanism of sexual selection contributes to the maintenance, and possibly even the establishment, of two almost discrete male morphs in the polyphenic black scavenger fly Sepsis thoracica (Diptera: Sepsidae): small and black, or large and amber. We performed two complementary laboratory experiments to evaluate the mating success of the different male morphs and the behaviors (of both males and females) presumably mediating their mating success. We found evidence for intraspecific disruptive sexual selection on male body size that is mediated by male-male interactions, and significant positive directional selection on body size that interacted with (directional) selection on coloration, likely contributing to the origin and/or maintenance of the threshold relationship between the two traits in this species. The simultaneous occurrence of disruptive selection and polyphenism in S. thoracica supports the role of sexual selection in the intraspecific diversification of coupled traits (here body size and coloration), which could be a speciation starting point.

性选择（sexual selection）包含两大核心组分——雌性偏好与雄性间竞争，二者可驱动雄性采取替代繁殖策略（alternative reproductive tactics）以最大化自身繁殖成功率。显著影响繁殖成功率的两类关键性状为体型大小与体色：前者可通过雄性间争斗帮助雄性获取交配机会，后者则可作为吸引雌性的视觉信号。本研究旨在探究，是否存在某类性选择机制参与维持乃至促成了多态黑腐食蝇Sepsis thoracica（双翅目: 黑秆蝇科）中两种近乎离散的雄性形态——小型黑化型与大型琥珀色型。我们开展了两项互补的室内实验，以评估不同雄性形态的交配成功率，并解析可能介导二者交配成功率差异的雌雄个体行为机制。研究结果显示，存在由雄性间互作介导的雄性体型种内破坏性性选择（intraspecific disruptive sexual selection）证据；同时体型受到显著的正向定向选择，且该选择与体色的定向选择存在互作效应，这可能促成了该物种种群中两大性状间阈值关联的起源与维持。S. thoracica中破坏性选择与多态现象的共存，佐证了性选择在耦合性状（本文中指体型与体色）的种内分化中发挥的关键作用，而这一过程或可成为物种形成的起始点。