In species with multiple mating, intense sexual selection may occur both before and after copulation. However, comparing the strength of pre‐ and postcopulatory selection is challenging, because (i) postcopulatory processes are generally difficult to observe and (ii) the often‐used opportunity for selection (I) metric contains both deterministic and stochastic components. Here, we quantified pre‐ and postcopulatory male fitness components of the simultaneously hermaphroditic flatworm, Macrostomum lignano. We did this by tracking fluorescent sperm—using transgenics—through the transparent body of sperm recipients, enabling to observe postcopulatory processes in vivo. Moreover, we sequentially exposed focal worms to three independent mating groups, and in each assessed their mating success, sperm‐transfer efficiency, sperm fertilizing efficiency, and partner fecundity. Based on these multiple measures, we could, for each fitness component, combine the variance (I) with the repeatability (R) in individual success to assess the amount of repeatable variance in individual success—a measure we call the repeatable opportunity for selection (I_R). We found higher repeatable opportunity for selection in sperm‐transfer efficiency and sperm fertilizing efficiency compared to mating success, which clearly suggests that postcopulatory selection is stronger than precopulatory selection. Our study demonstrates that the opportunity for selection contains a repeatable deterministic component, which can be assessed and disentangled from the often large stochastic component, to provide a better estimate of the strength of selection.

中文翻译：

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交配前和交配后健身组件的重复选择机会不同

在多次交配的物种中，交配前后都可能发生强烈的性选择。但是，比较交配前和交配后选择的强度具有挑战性，因为（i）交配后的过程通常很难观察到，并且（ii）经常选择的机会（I）度量标准既包含确定性要素，也包含随机性要素。在这里，我们量化了同时雌雄同体扁虫Macrostomum lignano的交配前和交配后男性健身成分。我们通过使用转基因技术通过透明的精子受体跟踪荧光精子来实现此目的，从而能够观察体内的交配后过程。此外，我们依次将蠕虫暴露于三个独立的交配组，并分别评估了它们的交配成功率，精子转移效率，精子受精效率和伴侣繁殖力。基于这些多种度量，我们可以针对每个适应度组件，将个体成功中的方差（I）与可重复性（R）结合起来，以评估个体成功中可重复方差的数量，该度量称为可重复选择机会（我_[R）。与交配成功相比，我们发现精子转移效率和精子受精效率的选择机会更高，这显然表明交配后的选择比交配前的选择更强大。我们的研究表明，选择的机会包含可重复的确定性成分，可以对这些确定性成分进行评估，并将其与通常较大的随机成分分开，以更好地估计选择的强度。