Table 1-9 from Fluctuating selection strength and intense male competition underlie variation and exaggeration of a water strider's male weapon

Sexually selected traits can reach high degrees of phenotypic expression and variation under directional selection. A growing number of studies suggest that such selection can vary in space, time and form within and between populations. However, the impact of these fluctuations on sexual trait evolution is poorly understood. In the water strider Microvelia longipes, males display striking trait exaggeration and phenotypic variation manifested as extreme differences in the rear leg length. To study the origin and maintenance of this exaggerated trait, we conducted comparative behavioural, morphometric and reaction norm experiments in a selection of Microvelia species. We uncovered differences both in the mating behaviour and the degree of sexual dimorphism across these species. Interestingly, M. longipes evolved a specific mating behaviour where males compete for egg-laying sites, consisting of small floating objects, to intercept and copulate with gravid females. Through male–male competition assays, we demonstrated that male rear legs are used as weapons to dominate egg-laying sites and that intense competition is associated with the evolution of rear leg length exaggeration. Field observations revealed rapid fluctuation in M. longipes habitat stability and the abundance of egg-laying sites. Paternity tests using genetic markers demonstrated that small males could only fertilize about 5% of the eggs when egg-laying sites are limiting, whereas this proportion increased to about 20% when egg-laying sites become abundant. Furthermore, diet manipulation and artificial selection experiments also showed that the exaggerated leg length in M. longipes males is influenced by both genetic and nutritional factors. Collectively, our results highlight how fluctuation in the strength of directional sexual selection, through changes in the intensity of male competition, can drive the exaggeration and phenotypic variation in this weapon trait.

在定向选择（directional selection）作用下，性选择性状（sexually selected traits）可达到极高的表型表达（phenotypic expression）程度与表型变异（phenotypic variation）水平。越来越多的研究表明，这类选择在种群内部及种群间可随空间、时间与作用形式发生变化。然而，学界对这类波动对性选择性状演化的影响仍知之甚少。在水黾（Microvelia longipes）中，雄性展现出显著的性状夸张化与表型变异，具体表现为后足长度存在极端差异。为探究该夸张性状的起源与维持机制，我们针对选取的短沟跳黾属（Microvelia）物种开展了比较行为学、形态测量学以及反应规范（reaction norm）实验。我们发现，这些物种在交配行为与性二态性（sexual dimorphism）程度上均存在差异。值得注意的是，长翅短沟跳黾（Microvelia longipes）演化出了一种特殊的交配行为：雄性会争夺由小型漂浮物体构成的产卵位点，以此拦截并与怀卵雌性交配。通过雄性间竞争实验，我们证实雄性后足可作为武器以夺取产卵位点，且激烈的雄性竞争与后足长度的夸张化演化密切相关。野外观察显示，长翅短沟跳黾的栖息地稳定性与产卵位点丰度会快速波动。利用遗传标记（genetic marker）进行的父权分析（paternity test）显示，当产卵位点有限时，体型较小的雄性仅能使约5%的卵受精；而当产卵位点充足时，这一比例可提升至约20%。此外，饮食操控实验与人工选择（artificial selection）实验同样证实，长翅短沟跳黾雄性的夸张后足长度同时受遗传与营养因素调控。综合来看，我们的研究结果揭示了定向性选择强度的波动如何通过改变雄性竞争强度，推动该武器性状的夸张化演化与表型变异。