The genetic basis of reproductive isolation through intragenomic conflict

物种可能是最重要的生物单位，同一物种的成员共享基因流和共同特征。了解物种是如何形成的，对于广泛理解生物学，特别是生物多样性是如何产生的至关重要。教科书上对一个物种如何分裂成两个物种的解释是“生态物种形成”：种群分化是因为当地生态的差异。经典的例子是达尔文的雀类，在一个岛屿上，鸟类大多吃小种子，并进化出脆弱的喙，很容易收集它们，而在另一个岛屿上，可以买到大坚果，种群进化出大喙，把它们打开。最终，种群之间的差异如此之大，以至于它们不能进行杂交。有许多这种生态物种形成的很好的例子，比如刺鱼和达尔文的雀类。但是，关于物种形成可能发生的主要替代理论，有一个主要的替代理论。这一理论被称为冲突物种形成，并认为被隔离的种群将在其自私的遗传元素上发生分歧。虽然大多数基因通常在个体内相互合作，以增加后代的数量，但自私的因素却通过操纵生殖来增加自己的成功，欺骗了他们基因组中的其他基因。这些基因引起的冲突被认为会迅速在关键生殖基因上造成种群之间的差异。这使得种群之间的杂交更难生存和繁殖--对雄性精子的产生尤其有害。冲突物种形成理论认为，新物种的形成是由于基因组内部的这些冲突，而不是外部生态因素。如果发现强有力的证据支持冲突物种形成，我们将不得不从根本上重新思考我们关于物种形成如何以及为什么发生的许多想法。冲突物种形成得到了大量间接证据的支持，但缺乏密切相关种群之间这一过程的关键证据。在这个项目中，我们将研究这一过程被认为正在进行的一对种群。我们发现，北非果蝇可以与欧洲种群进行愉快的杂交。然而，北非的一些D.subbobscura携带一种名为“SRS”的自私的X染色体，这种X染色体在雄性体内杀死了他所有的Y染色体精子，所以他的后代都继承了SRS染色体，并且都是女儿。由于这种自私的X会遗传给几乎所有的后代，而正常的X染色体只会遗传给一半的男性后代，自私的X应该会在人群中迅速传播。然而，SRS X不能传播到欧洲，因为携带SRS的杂交雄性是不育的，可能是因为他们的精子杀死机制发生故障，杀死了他们所有的精子。自私的染色体在原本完全相容的种群之间造成不相容的独特例子，为我们提供了一个研究冲突物种形成的早期阶段的机会。在北非，一些携带SRS的男性确实会生下一些儿子。我们认为这是因为基因组的其余部分已经进化到抑制SRS杀死Y精子，从而产生了一些儿子。我们将首先通过对高繁殖力和低繁殖力的杂交进行测序，确定导致SRS杂交不育的基因，从而检验冲突物种形成理论。然后，我们将通过确定这些基因是否与提供抗驱力的基因相同来测试冲突物种形成理论。我们还将检查这些基因在北非的进化是否比西班牙更快，这表明这种差异是由非洲存在的SRS造成的，而不是两个种群随着时间的推移只是在遗传上分化。

Species are perhaps the most vital biological unit, with members of the same species sharing gene flow and common traits. Understanding how species are formed is critical for a broad understanding of biology, particularly of how biodiversity is created. The textbook explanation for how one species splits into two is "ecological speciation": populations diverge because of differences in local ecology. The classic example is Darwin's finches, where on one island birds mostly eat small seeds, and evolve delicate bills to easily gather them, while on another island large nuts are available, and the population evolves large beaks to crack them open. Eventually the differences between the populations are so great that they cannot interbreed. There are many good examples of this kind of ecological speciation, such as stickleback fish and Darwin's finches.However, there is a major alternative theory about how speciation might occur. This theory is called conflictual speciation, and holds that populations that are isolated will diverge in their selfish genetic elements. While most genes typically cooperate within an individual to increase the number of offspring produced, selfish elements instead manipulate reproduction to increase their own success, cheating the other genes in their genome. The conflict these genes cause is thought to rapidly create differences between populations in key reproductive genes. This makes hybrids between populations less able to survive and breed -and is particularly damaging to sperm production in males. Conflictual speciation theory suggests that new species form due to these internal conflicts within the genome, rather than external ecological factors. If strong evidence is found to support conflictual speciation, we will have to fundamentally rethink many of our ideas about how and why speciation occurs.Conflictual speciation has been supported by a wealth of circumstantial evidence, but crucial evidence of the process in action between closely related populations is lacking. In this project, we will study a pair of populations where this process is thought to be ongoing. We have found that North African Drosophila subobscura flies can interbreed happily with European populations. However, some D. subobscura in North Africa carry a selfish X chromosome called "SRs", which in males kills all his Y-chromosome sperm, so all his offspring inherit the SRs chromosome and are daughters. As this selfish X is passed on to almost all offspring, while normal X chromosomes are only passed to half a male's offspring, the selfish X should spread rapidly through populations. However, the SRs X cannot spread into Europe because hybrid males that carry SRs are sterile, probably because their sperm killing mechanism malfunctions and kills all their sperm. This unique example of a selfish chromosome creating incompatibilities between populations that are otherwise completely compatible, provides us with an opportunity to study the early stages of conflictual speciation. In North Africa, some males that carry SRs do produce some sons. We think this is because the rest of the genome has evolved to suppress the killing of Y sperm by SRs, allowing some sons to be produced. We will test the theory of conflictual speciation by first determining the genes that cause the infertility in the SRs hybrids by sequencing hybrids with high and low fertility. We will then test the theory of conflictual speciation by determining whether these genes are the same ones that provide resistance to drive. We will also examine whether these genes have been evolving more rapidly in North Africa than Spain, which would suggest that the difference is caused by SRs being present in Africa, rather than both populations simply diverging genetically over time.