Data from: Sperm competitive ability evolves in response to experimental alteration of operational sex ratio

In naturally polygamous organisms such as Drosophila, sperm competitive ability is one of the most important components of male fitness and is expected to evolve in response to varying degrees of male–male competition. Several studies have documented the existence of ample genetic variation in sperm competitive ability of males. However, many experimental evolution studies have found sperm competitive ability to be unresponsive to selection. Even direct selection for increased sperm competitive ability has failed to yield any measurable changes. Here we report the evolution of sperm competitive ability (sperm defense-P1, offense-P2) in a set of replicate populations of Drosophila melanogaster subjected to altered levels of male–male competition (generated by varying the operational sex ratio) for 55–60 generations. Males from populations with female-biased operational sex ratio evolved reduced P1 and P2, without any measurable change in the male reproductive behavior. Males in the male-biased regime evolved increased P1, but there was no significant change in P2. Increase in P1 was associated with an increase in copulation duration, possibly indicating greater ejaculate investment by these males. This study is one of the few to provide empirical evidence for the evolution of sperm competitive ability of males under different levels of male–male competition.

在天然多配偶生物（例如果蝇（Drosophila））中，精子竞争能力是雄性适合度的核心组分之一，其演化应随雄性间竞争程度的波动而发生适应性改变。已有多项研究证实，雄性的精子竞争能力存在丰富的遗传变异。然而，诸多实验进化研究却发现，精子竞争能力对选择压力并无响应；即便直接针对提升精子竞争能力开展选择实验，也未能观测到任何可量化的演化变化。本研究针对黑腹果蝇（Drosophila melanogaster）的一组重复种群，通过调整有效性比来调控雄性间竞争水平，历经55至60个世代的选择后，报道了其精子竞争能力（精子防御能力P1、精子进攻能力P2）的演化特征。来自雌性偏向有效性比种群的雄性个体，其P1与P2值均出现显著下降，而雄性生殖行为未发生可量化的改变；处于雄性偏向处理组的雄性个体，其P1值有所提升，但P2未出现显著变化。P1值的提升与交配时长的延长呈显著关联，这或暗示此类雄性的射精投入有所增加。本研究是少数几项为不同雄性间竞争水平下雄性精子竞争能力的演化提供实证依据的研究之一。