Quantifying fitness benefits of polyandry in Atlantic salmon

为什么这么多物种的雌性与多个雄性交配，除了精子之外，它们从这种一夫多妻中得不到任何好处，而且只与一个雄性交配就足以确保完全生育？这在进化生物学中仍然是一个长期存在的问题，因为当雌性交配时，人们已经测量到了它们的巨大成本。我们现在知道，在交配和受精之间的配子水平上运行的机制比之前假设的要复杂得多。在一夫多妻制下，随着更大和更多样化的精子群体竞争受精，甚至有更多的选择性机会来适应，以提高精子的竞争能力，和/或神秘的雌性选择。最近的实验研究表明，一夫多妻制允许雌性在配子水平上为她们产生的后代‘捕捉’遗传利益。这样的好处可能是因为精子质量和一般健康之间的联系，或者是因为雌性已经进化出了一种机制，允许选择携带固有优势基因或未受损DNA的精子，或者可能存在携带与雌性/卵子基因更相容的基因的精子的隐晦选择。我们建议在一个系统中量化一夫多妻制的好处，该系统使用仔细控制的体外受精，从同一个女性产生后代批次，只有根据他们是由五个单独的男性受精，还是由相同的五个男性受精，但同时进行精子竞争。我们的实验模型将是大西洋鲑鱼，在大西洋鲑鱼中，雌性鱼一次最多与16只雄性鱼自然交配，体外受精允许我们进行体外受精，控制或消除对后代健康的许多重要混杂影响，例如对卵子质量的偏向或可变的母体投资。我们之前在鲑鱼体外受精动力学方面的工作将使我们能够确保受精率不会成为每批卵子的限制因素。对于每个女性，我们将创建5个卵子批次，这些卵子是由5个不同的单身男性的精子受精(=单性繁殖条件)，以及5个卵子批次是由来自相同5个男性的匀浆混合精子受精(=一夫多妻)。这些配对的比较将重复25次，包括25名女性和125名男性。该设计将控制成体身份和卵子的影响，并产生仅根据是否存在精子竞争和/或卵子选择而不同的受精后代批次。受精后，将测量后代批次在胚胎发育、卵孵化、绵羊蛋白发育、鱼苗出苗以及PARR存活、成功和生长等关键阶段的存活和生长。这些健康措施都将在实验溪流段进行，允许进行与环境相关的选择。使用挪威的IMS鲑鱼野外站将提供能够进行实验控制的设施，同时在半自然条件下饲养卵和鱼苗，并从许多处理鲑鱼卵、胚胎、鱼苗和鱼苗的正常力量中进行选择。我们必须在自然环境中进行选择的条件下测量蛋和后代的表现，因为这是健康益处最重要的地方。在项目结束时，我们将测量在一夫多妻制下可以获得哪些健身益处，以及这些益处发生在鲑鱼的哪个生命阶段。这一结果对于纯粹理解一夫多妻制的遗传益处将是至关重要的。重要的是，我们的工作将应用于鲑鱼孵化场的繁殖策略。为了保护遗传多样性，孵化场协议现在使用强制一夫一妻制，以最大限度地增加参与繁殖的雄性繁殖者的数量。然而，这些协议可能排除了一夫多妻制的重要好处，我们的项目将衡量这些好处可能有多重要。

Why do females of so many species mate with multiple males, when they gain nothing from this 'polyandry' apart from spermatozoa, and mating with only one male is sufficient to ensure full fertility? This remains an enduring question in evolutionary biology, because significant costs have been measured for females when they multiply mate. We now know that the mechanisms operating at the gamete level between mating and fertilization are much more complex than previously supposed. Under polyandry, with bigger and more diverse populations of spermatozoa competing for fertilization, there is even more selective opportunity for adaptations that promote sperm competitive ability, and / or cryptic female choice. Recent experimental research suggests that polyandry allows females to 'capture' genetic benefits at the gamete level for their resulting offspring. Such benefits may arise because of links between sperm quality and general fitness, or because females have evolved mechanisms that allow selection of sperm that carry intrinsically superior genes or undamaged DNA, or there may be cryptic choice for sperm that carry genes that are more compatibile to the female/egg genotype. We propose to quantify benefits of polyandry in a system using carefully controlled in vitro fertilizations to create offspring batches from the same female that only differ according to whether they were fertilized by five separate males singly, or by the same five males but in simultaneous sperm competition. Our experimental model will be the Atlantic salmon, in which females naturally mate with up to 16 males at once, and where external fertilization allows us to conduct in vitro fertilizations that control or eliminate a number of important confounding effects on offspring fitness, such as biased or variable maternal investment in egg quality. Our previous work on salmon in vitro fertilization dynamics will allow us to ensure that fertility is not a limiting factor for each egg batch. For each female, we will create 5 egg batches which were fertilized by sperm from 5 different, single males (=monandrous condition), and 5 egg batches that were fertilized by homogenised mixes of sperm from the same 5 males (= polyandry condition). These paired comparisons will be repeated 25 times, using 25 females and 125 males. The design will control for adult identity and egg effects, and generate offspring batches that only differ according to fertilization in the presence or absence of sperm competition and/or egg choice. After fertilization, the offspring batches will be measured for survival and growth across key stages of embryo development, egg hatch, alevin development, fry emergence, and parr survival, success and growth. These fitness measures will all be conducted in experimental stream sections that allow environmentally-relevant selection to operate. Use of the Ims salmon field station in Norway will provide access to facilities that enable experimental control, while rearing eggs and fry in semi-natural conditions with selection from many of the normal forces that operate on salmon eggs, embryos, fry and parr. It is essential that we measure egg and offspring perfomance under conditions that present some selection from the natural environment, because this is where fitness benefits will be most important. At the conclusion of the project, we will have measured what fitness benefits can be gained under polyandry, and what life stages these gains occur in salmon. The results will be of central importance to a pure understanding of the genetic benefits of polyandry. Importantly, our work will have applied relevance for salmon hatchery breeding strategies. To conserve genetic diversity, hatchery protocols now use enforced monogamy to maximize the number of male breeders involved in reproduction. However, there may be important polyandry benefits that are excluded by these protocols, and our project will measure how important these benefits might be.