Data from: Delineating the roles of males and females in sperm competition

Disentangling the relative roles of males, females and their interactive effects on competitive fertilization success remains a challenge in sperm competition. In this study, we apply a novel experimental framework to an ideally suited externally fertilizing model system in order to delineate these roles. We focus on the chinook salmon, Oncorhynchus tshawytscha, a species in which ovarian fluid (OF) has been implicated as a potential arbiter of cryptic female choice for genetically compatible mates. We evaluated this predicted sexually selected function of OF using a series of factorial competitive fertilization trials. Our design involved a series of 10 factorial crosses, each involving two ‘focal’ rival males whose sperm competed against those from a single ‘standardized’ (non-focal) rival for a genetically uniform set of eggs in the presence of OF from two focal females. This design enabled us to attribute variation in competitive fertilization success among focal males, females (OF) and their interacting effects, while controlling for variation attributable to differences in the sperm competitive ability of rival males, and male-by-female genotypic interactions. Using this experimental framework, we found that variation in sperm competitiveness could be attributed exclusively to differences in the sperm competitive ability of focal males, a conclusion supported by subsequent analyses revealing that variation in sperm swimming velocity predicts paternity success. Together, these findings provide evidence that variation in paternity success can be attributed to intrinsic differences in the sperm competitive ability of rival males, and reveal that sperm swimming velocity is a key target of sexual selection.

在精子竞争（sperm competition）研究领域，厘清雄性、雌性及其交互效应对竞争性受精成功率的相对作用始终是一项核心挑战。本研究针对一类极为适配的体外受精模式生物，采用全新的实验框架以解析上述三类因素的具体作用。我们选取大鳞大麻哈鱼（Oncorhynchus tshawytscha）作为研究对象，该物种的卵巢液（ovarian fluid, OF）已被证实可作为隐秘雌性选择（cryptic female choice）遗传相容性配偶的潜在调控介质。
我们通过一系列析因竞争性受精实验（factorial competitive fertilization trials），对卵巢液的这一预测性性选择功能开展了验证。本实验设计包含10组析因杂交组合：每组均纳入2名“焦点”竞争雄性，其精子与1名“标准化”（非焦点）对照雄性的精子展开竞争，并以2名焦点雌性的卵巢液作用于一组遗传均一的卵。该实验设计可使我们将竞争性受精成功率的变异归因于焦点雄性、雌性（卵巢液）及其交互效应的差异，同时控制了对照雄性精子竞争能力差异以及雌雄基因型交互作用所引入的变异。
借助该实验框架，我们发现精子竞争力的变异仅来源于焦点雄性的内在精子竞争能力差异；后续分析进一步证实，精子游泳速度能够有效预测父权成功率，这一结果也为前述结论提供了支撑。综上，本研究结果表明，父权成功率的变异可归因于竞争雄性内在的精子竞争能力差异，同时揭示了精子游泳速度是性选择的关键作用靶点。