Molecular mechanisms and evolutionary origins of teratozoospermia and sex ratio skewing in mice with Y chromosome deletions

当精子使卵子受精时，所产生的后代的性别取决于精子携带的是X染色体还是Y染色体。正常情况下，这是50%的几率，因此一半的后代是男性，一半是女性。然而，性染色体上的基因有可能表现出自私的行为，以增加它们被遗传的可能性：例如，如果携带X染色体的精子分泌一种毒素，杀死带有Y染色体的精子，或者使它们在受精卵子方面效率降低。这就是众所周知的基因组竞争。X染色体上有更多拷贝的这种“畸变者”基因传播得更快，导致这些基因在X染色体上的拷贝数量增加。如果X和Y染色体之间出现这样的基因组竞争，那么自然选择将有利于携带与X连锁的扭曲基因的作用相反的抑制基因的Y染色体的生存。这就导致了一场军备竞赛，放大了X和Y染色体上的竞争基因。在老鼠身上似乎就是这种情况。Y染色体上有几个不同的基因家族，每个基因都有几十个拷贝。当这些基因被删除时，携带该基因的雄性后代的性别比例就会出现偏差。在我们最近的工作中，我们已经鉴定了几个这样的Y染色体基因家族，并表明当Y染色体抑制基因缺失时，特定的X染色体基因被打开。这种X染色体基因的过度表达导致精子头部畸形，这在受精卵子方面效率较低。在能够使卵子受精的精子中，超过50%是携带X染色体的精子，这导致下一代的性别比例偏向于女性。我们提出的项目旨在了解这种基因组竞争是如何在小鼠的进化史中产生的分子基础，扭曲的基因是如何在之后传播的，以及它们是如何被阻遏基因的进化所控制的。为了发现这一点，我们将使用抗体进行特殊的染色，以显示X编码蛋白在精子中的位置，并利用这一点来研究它们可能如何影响精子头部的形状。看看X-精子和Y-精子在蛋白质位置上是否有差异，这将是特别有趣的。我们还将制造专门过度表达一个或多个候选X基因的转基因小鼠，并检查转基因品系中的精子形状异常和/或性别比例偏差。我们将更详细地研究Y连锁的抑制基因，看看它们是如何抵消扭曲基因的。一种名为Fly的基因非常有趣，因为它是双向转录的。转录是制作活性基因的“工作拷贝”(信使核糖核酸拷贝)的过程。如果一个基因是双向转录的，产生的mRNAs可以在一个称为RNA抑制(RNAi)的过程中使相关基因沉默。我们将测试RNAi是否是让Y染色体抑制基因抵消X染色体扭曲基因的机制。最后，我们将看看在一系列不同的小鼠物种的X和Y染色体上，每个候选基因有多少个副本。这将告诉我们基因组冲突是什么时候开始的。通过比较冲突开始前后基因的序列和活动模式，我们或许能够找到导致冲突的特定突变。更好地理解性别比扭曲及其背后的进化过程，不仅在纯科学方面很重要，而且在潜在的经济方面也很重要。例如，它可能会让我们在牛等农场动物物种中发现相互冲突的基因，从而允许选择性繁殖优先产生雌性小牛的公牛。此外，对扭曲基因影响的途径有了更全面的生物化学理解，我们就可以直接针对这些途径来影响后代的性别比。

When a sperm fertilizes an egg, the sex of the resulting offspring is determined by whether the sperm carries an X or a Y chromosome. Normally, this is a 50:50 chance, and thus half the offspring are male and half are female. However, there is the possibility that genes on the sex chromosomes can act 'selfishly' to increase their likelihood of being passed on: for example if X-bearing sperm secrete a toxin that kills Y-bearing sperm, or makes them less effective at fertilising the egg. This is known as genomic competition. X chromosomes with more copies of such a 'distorter' gene spread more rapidly, leading to an increase in the copy number of these genes on the X chromosome. If such a genomic competition arises between the X and Y chromosomes, then natural selection will favour the survival of Y chromosomes which carry a 'repressor' gene opposing the action of the X-linked distorter gene. This then leads to an arms race, amplifying the competing genes on both X and Y chromosomes. This seems to be the case in mouse. There are several different families of genes on the Y chromosome which are present in many dozens of copies each. When these genes are deleted, there is a sex ratio skew in the offspring of the males carrying the deletion. We have identified several of these Y chromosome gene families in our recent work, and have shown that specific X chromosome genes are switched on when the Y chromosomal repressor genes are deleted. This 'overexpression' of the X chromosome genes leads to sperm with malformed heads, which are less effective at fertilising eggs. Of the sperm that are able to fertilise eggs, more than 50% are X-bearing sperm, leading to the sex ratio skew in favour of females in the next generation. Our proposed project is aimed at understanding the molecular basis of how this genomic competition arose during the evolutionary history of the mouse, how the distorter genes spread thereafter, and how they have been contained by the evolution of the repressor genes. In order to discover this, we will make special stains using antibodies to show where the X-encoded proteins are located within the sperm, and use this to work out how they may affect sperm head shape. It will be especially interesting to see if there is a difference between X-bearing and Y-bearing sperm in terms of protein location. We will also make transgenic mice that specifically overexpress one or more of the candidate X genes, and check for sperm shape abnormalities and/or a sex ratio skew in the transgenic lines. We will look in closer detail at the Y-linked repressor genes to see how they counteract the distorter genes. One gene, called Fly, is very interesting as it is transcribed in both directions. Transcription is the process that makes 'working copies' (mRNA copies) of active genes. If a gene is transcribed in both directions, the resulting mRNAs can silence related genes in a process called RNA inhibition (RNAi). We will test whether RNAi is the mechanism that lets the Y chromosomal repressor genes counteract the X chromosomal distorter genes. Finally, we will see how many copies of each of the candidate genes are present on the X and Y chromosomes of a range of different mouse species. This will tell us when the genomic conflict started. By comparing the sequence and activity patterns of genes before and after the start of the conflict, we may be able to find specific mutations that caused the conflict. A better understanding of sex ratio distortion, and the evolutionary processes behind it, is important not only in pure science terms, but also potentially in economic terms. For example, it may allow us to discover conflicting genes in farm animal species such as cattle, allowing selective breeding for bulls that preferentially generate female calves. Also, a fuller biochemical understanding of the pathways affected by the distorter genes could let us directly target these pathways in order to affect offspring sex ratio.