Differential fertilisation compatibility in Atlantic salmon: implications for farmed salmon gene introgression and hybridisation

不同物种间繁殖最重要的障碍之一是精子和卵子的相容性。生物学家正试图了解配子之间的识别和相容性的进化过程，但对于大多数物种来说，精子和卵子是如何共同进化以识别彼此以受精的，我们一无所知。最近的研究表明，这一过程可能发展得非常快，可能是因为雄性之间的精子竞争使卵子受精，以及雄性和雌性之间为了从繁殖中获得最大利益而发生的冲突。我们将研究大西洋鲑鱼的精子和卵子之间的兼容性，这是一个理想的模型，从纯粹和应用的角度来研究这些问题。成年鲑鱼回到它们孵化的河流中繁殖。这种策略意味着不同种群之间的遗传混合很低，生物学家已经表明，不同的菌株具有明显的遗传差异，并且已经进化出适合自己种群的特定适应（如大小、产卵年龄等）。由于刚刚提到的那些快速进化过程，这些隔离条件使鲑鱼成为一个可能在精子和卵子兼容性方面进化出差异的物种。我们已经发现了鲑鱼品系之间受精兼容性差异的证据，这是基于非常具体但重要的免疫基因。在这个项目中，我们计划通过观察精子和卵子受精兼容性在不同种群之间的变化来更广泛地研究这些差异，同时也将这些理论应用于野生大西洋鲑鱼面临的一个关键问题。由于许多原因，野生大西洋鲑鱼是一种重要的鱼类，但不幸的是，它们的数量正在严重减少。野生鲑鱼种群面临的一个主要问题是养殖鲑鱼的逃逸，这在鲑鱼的活动范围内发生了非常高的水平。逃出来的鱼可以存活下来，有些还能进入野生鲑鱼的产卵地。在这里，他们试图与野生鱼类一起产卵，这带来了一个主要问题：养殖鲑鱼已经被驯化，因为它们与野生鱼类存在巨大的基因差异。养殖和野生鲑鱼繁殖的后代携带的养殖基因较差，这意味着不同种群中重要的野生基因会被稀释。不断释放这么多的养殖鱼类最终可能会在基因上淹没野生基因，可能永远失去特别进化的地方适应能力。人们可以想象，如果家养狗不断被释放到狼群中繁殖，也会出现类似的情况；如果每年以足够高的速度释放，最终纯粹的野狼基因将会丢失。了解养殖鲑鱼问题程度的一个重要信息是，养殖鲑鱼和野生鲑鱼杂交的繁殖能力如何？驯化可能已经改变了养殖鲑鱼的生育能力，或提高或降低。研究表明，养殖鱼类可以成功地与野生鱼类繁殖，但它们的繁殖能力较差。然而，养殖鱼类究竟有多差，在不同的野生种群繁殖时，这种差是如何变化的？这些信息对于了解养殖鱼类对野生种群的风险非常重要。我们将通过测量人工养殖的精子和卵子与野生鲑鱼精子和卵子的生育能力来回答这个问题。我们已经开发了在受控条件下进行受精的试验，但它模仿了鲑鱼河中精子和卵子的正常功能。我们还将检查野生和养殖精子混合并竞争一批卵子时的受精成功率，这一过程通常发生在野生环境中。我们还将测量养殖鲑鱼与近亲鳟鱼的繁殖能力，因为这是生产养殖杂交鲑鱼的另一种可能方式。我们的研究将使我们能够测量养殖鲑鱼的相对繁殖力，从而为政府平衡野生鱼类保护与商业上重要的鲑鱼养殖提供非常有用的信息。

One of the most important barriers to reproduction between different species is compatibility between sperm and egg. Biologists are trying to understand the evolution of recognition and compatibility between gametes, but for most species nothing is known about how sperm and egg co-evolve to recognise one-another for fertilization. Recent research shows that this process probably evolves very quickly, probably because of sperm competition between males to fertilise the eggs, and conflict between males and females to get the most out of reproduction. We will examine the compatibility between sperm and egg in Atlantic salmon, which is an ideal model to examine these questions for both pure and applied reasons. Salmon adults return to the river where they hatched to reproduce. This strategy means that genetic mixing between different populations is low, and biologists have shown that different strains have clear genetic differences, and have evolved specific adaptations for their own population (such as size, spawning age etc). These isolating conditions make salmon a likely species to have evolved differences between populations in sperm and egg compatibility, because of those rapid evolutionary processes just mentioned. We have found evidence for differences in fertilization compatibility between salmon strains, based on quite specific, but important immunity genes. In this project we plan to examine these differences in much more breadth by looking at how sperm and egg fertilization compatibility has changed between different populations, while also applying these theories to a key problem facing wild Atlantic salmon. Wild Atlantic salmon are an important fish for many reasons, but are unfortunately in severe decline. A major problem facing wild salmon stocks is the escape of farmed salmon, which occurs over the salmon's range at a very high level. Escaped fish can survive, and some make it onto wild salmon spawning grounds. Here they attempt to spawn with wild fish, presenting a major problem: farmed salmon have been domestically-selected for big genetic differences with wild fish. Offspring from farmed and wild salmon reproduction carry inferior farmed genes, and mean that important wild genes in different populations become diluted. Continual release of so many farmed fish could eventually genetically swamp wild genes, possibly permanently losing specifically evolved local adaptations forever. One could imagine a similar situation if domestic dogs were continually released into wolf populations to breed; eventually the pure, wild wolf genes would be lost if release occurred every year at high enough rates. A vital piece of information in understanding the level of the farmed salmon problem is how fertile are crosses between farmed and wild salmon? Domestication might have altered farmed salmon fertility, either up or down. Research has shown that farmed fish can successfully reproduce with wild fish, but that they are inferior at reproduction. However, exactly HOW inferior are farmed fish, and how does this inferiority vary when reproducing with different wild populations? This information is important to understand the risk of farmed fish to wild stocks. We will answer this question by measuring the fertility of farmed sperm and eggs with wild salmon sperm and eggs. We have developed trials that run fertilisations in controlled conditions, but which mimic the normal function of sperm and egg in a salmon river. We will also examine fertilisation success when wild and farmed sperm mix and compete for a batch of eggs, a process which occurs normally in the wild. We will also measure how fertile farmed salmon are with a close relative, trout, since this is another possible way for farmed hybrids to be produced. Our research will allow us to measure the relative fertility of farmed salmon, thus providing very useful information for governments balancing wild fish conservation with commercially important salmon farming.