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

不同物种之间繁殖的最重要障碍之一是精子和卵子之间的兼容性。生物学家正试图了解配子之间的识别和兼容性的进化，但对于大多数物种来说，关于精子和卵子是如何共同进化来识别彼此以进行受精的，我们一无所知。最近的研究表明，这一过程可能进化得非常快，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最大限度地利用生殖而发生的冲突。我们将研究大西洋鲑鱼的精子和卵子之间的兼容性，这是一个无论是出于纯粹的原因还是应用的原因来研究这些问题的理想模型。成年鲑鱼回到河里孵化繁殖。这一策略意味着不同种群之间的遗传混合率很低，生物学家已经证明，不同的菌株具有明显的遗传差异，并为自己的种群(如大小、产卵年龄等)进化出特定的适应。这些隔离条件使得鲑鱼很可能是种群之间在精子和卵子兼容性方面存在进化差异的物种，因为刚才提到的那些快速进化过程。我们已经发现了鲑鱼品系之间受精亲和性差异的证据，这是基于非常特定但重要的免疫基因。在这个项目中，我们计划通过观察不同种群之间精子和卵子受精兼容性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。出于许多原因，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下降。野生三文鱼种群面临的一个主要问题是养殖三文鱼的逃逸，这种情况发生在三文鱼的活动范围内，水平非常高。逃脱的鱼可以存活下来，有些还能逃到野生鲑鱼产卵场。在这里，他们试图与野生鱼类产卵，这带来了一个主要问题：养殖的三文鱼因与野生鱼类存在巨大的遗传差异而被国内选中。养殖和野生鲑鱼繁殖的后代携带劣质养殖基因，这意味着不同种群中重要的野生基因被稀释。持续释放如此多的养殖鱼类最终可能会在基因上淹没野生基因，可能永远失去特定进化的局部适应能力。如果家犬不断地被释放到狼群中繁殖，人们可以想象类似的情况；如果每年以足够高的速度释放，最终纯净的野狼基因将会丢失。在理解养殖三文鱼问题的水平方面，一个至关重要的信息是，养殖三文鱼和野生三文鱼杂交的肥力有多大？驯化可能改变了养殖三文鱼的繁殖力，无论是向上还是向下。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，养殖的鱼到底有多差，当与不同的野生种群繁殖时，这种劣势有何不同？这一信息对于了解养殖鱼类对野生种群的风险很重要。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，但这些试验模拟了鲑鱼河中精子和卵子的正常功能。我们还将检查野生精子和养殖精子混合并竞争一批卵子的受精成功与否，这一过程在野生环境中通常会发生。我们还将测量养殖鲑鱼与近亲鲑鱼的肥沃程度，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三文鱼的相对繁殖力，从而为政府平衡野生鱼类保护和重要的商业三文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.