Post-copulatory opportunities for sperm competition and cryptic female choice provide no offspring fitness benefits in externally fertilizing salmon.

There is increasing evidence that females can somehow improve their offspring fitness by mating with multiple males, but we understand little about the exact stage(s) at which such benefits are gained. Here, we measure whether offspring fitness is influenced by mechanisms operating solely between sperm and egg. Using externally fertilizing and polyandrous Atlantic salmon (Salmo salar), we employed split-clutch and split-ejaculate in vitro fertilization experiments to generate offspring using designs that either denied or applied opportunities for sperm competition and cryptic female choice. Following fertilizations, we measured 140 days of offspring fitness after hatch, through growth and survival in hatchery and near-natural conditions. Despite an average composite mortality of 61%, offspring fitness at every life stage was near-identical between groups fertilized under the absence versus presence of opportunities for sperm competition and cryptic female choice. Of the 21 551 and 21 771 eggs from 24 females fertilized under monandrous versus polyandrous conditions, 68% versus 67.8% survived to the 100-day juvenile stage; sub-samples showed similar hatching success (73.1% versus 74.3%), had similar survival over 40 days in near-natural streams (57.3% versus 56.2%) and grew at similar rates throughout. We therefore found no evidence that gamete-specific interactions allow offspring fitness benefits when polyandrous fertilization conditions provide opportunities for sperm competition and cryptic female choice.

越来越多的证据表明，雌性通过与多个雄性交配，在某种程度上能够提高其后代的适应性，但我们对获得这些益处的确切阶段知之甚少。在此，我们探究后代适应性是否受到仅在精子与卵子之间起作用的机制的影响。我们利用体外受精且具有一妻多夫制特点的大西洋鲑（Salmo salar），采用分割卵块和分割射精的体外受精实验，通过设计排除或提供精子竞争及雌性隐性选择机会的方式来培育后代。受精后，我们在孵化场及近乎自然的条件下，通过测量孵化后140天内后代的生长和存活情况来评估其适应性。尽管平均综合死亡率为61%，但在不存在与存在精子竞争及雌性隐性选择机会这两种情况下受精产生的各组后代，在每个生命阶段的适应性几乎相同。在单配与多配条件下，来自24只雌性的21551枚和21771枚卵，分别有68%和67.8%存活到100日龄幼鱼阶段；子样本显示出相似的孵化成功率（分别为73.1%和74.3%），在近乎自然的溪流中40天内的存活率也相近（分别为57.3%和56.2%），且整个过程中的生长速率也相似。因此，我们没有发现证据表明，当多配受精条件为精子竞争和雌性隐性选择提供机会时，特定配子间的相互作用能给后代适应性带来益处。