Expression of the chicken W chromosome and the evolution of female phenotypes

为什么大多数动物的雄性和雌性的外表和行为都不同？其中许多差异是遗传的，最终是由性染色体上的基因启动的。鸟类有一个雌性异性配对的性染色体系统，雌性有一条Z和W染色体，雄性有两条Z染色体。鸟类的W染色体，因为它是从母亲传给女儿的，在雄性身上从来不存在，只经历了雌性功能的选择，因此有可能在雌性健康中发挥重要作用。然而，染色体在过去一直被忽视，因为人们错误地认为它几乎没有活性基因，而且很难对其进行测序。我自己的初步基因表达数据表明，少数已知的W连锁基因是活跃表达的，这直接与之前关于W染色体在遗传上不活跃的假设相矛盾。因此，鸟类W染色体的基因含量具有很大的潜在进化和基因组学意义，但目前的鸡W染色体草案大部分是不完整的，而另一只来自斑雀的鸟类全基因组序列不包含W组装。鸡种为测试W染色体在雌性健康性状中的作用提供了一个完美的天然实验室，因为育种实践已经产生了为雌性特有的特征(产卵)和一系列性别二型性而选择的品种。我打算利用这种女性特征的多样性来揭示W染色体在女性适应能力进化中的作用。首先，我将利用基因表达数据来确定W染色体的编码内容。然后，我将测量已知的和新发现的W连锁基因在六个不同品种的鸡身上的表达，这些鸡种跨越了雌性表型的范围。这将使人们有可能确定W上那些在女性进化中发挥重要作用的基因。同时，我将利用新发现的W基因来研究鸟类性染色体的进化和分化。这个项目的结果将与四个不同的人群相关。首先，W染色体编码内容的鉴定将对基因组学社区有用，并将有助于进一步组装鸡基因组序列。其次，那些对性别差异的遗传和基因组基础感兴趣的人将能够整合这项研究中产生的信息；这包括对性别和疾病易感性的关系日益认识到的医学遗传学社区。第三，生育力下降是一个日益严重的问题，有关鸡的产蛋率的信息可能被证明对人类生育有用。第四，任何对分子进化感兴趣的人都会对鸟类性染色体的进化有兴趣，特别是因为雌性-异性恋遗传系统与哺乳动物和果蝇中更常见的雄性-异性恋性染色体形成了有益的对比。最后，我认为关于女性基础的知识对广大社区来说是有意义的。

Why do males and females in most animals look and act differently? Many of these differences are genetic, and ultimately are initiated by genes on the sex chromosomes. Birds have a system of female heterogametic sex chromosomes, with the female having a Z and a W chromosome, and males having two Z chromosomes. The avian W chromosome, because it is passed from mother to daughter and is never present in males, only experiences selection for female functions, and therefore has the potential to play a large role in female fitness. However, the chromosome has been ignored in the past due to a misconception that it has few active genes, and difficulties in sequencing it. My own preliminary gene expression data indicate that the few known W-linked genes are actively expressed, directly contradicting previous assumptions that the W chromosome was genetically inactive. The gene content of the avian W chromosome is therefore of great potential evolutionary and genomic interest, yet the current draft of the chicken W is largely incomplete, and the other avian full genome sequence, from the zebra finch, does not contain a W assembly. Chicken breeds present a perfect natural laboratory for testing the role of the W chromosome in female fitness traits, as breeding practices have produced breeds selected for female-specific traits (egg-laying), and a range of sexual dimorphisms. I intend to leverage this diversity of female traits in order to uncover the role of the W chromosome in the evolution of female fitness. First, I will employ gene expression data to identify the coding content of the W chromosome. I will then measure the expression of the known and newly-identified W-linked genes in six different breeds of chicken that span the range of female phenotypes. This will make it possible to identify those genes on the W that play an important role in female evolution. In parallel, I will use the newly identified W genes in order to study the evolution and divergence of the avian sex chromosomes. The outcomes of this project will be relevant to four different groups of people. First, the identification of the W chromosome coding content will be useful to the genomics community, and will aid in further assemblies of the chicken genome sequence. Second, those interested in the genetic and genomic basis of sex differences will be able to integrate the information generated in this study; this includes the medical genetics community where there is a growing realization of the relationship of sex and disease susceptibility. Third, declining fertility is an increasing problem, and information about egg-production rates in chickens may prove useful for human fertility. Fourth, knowledge about the evolution of the avian sex chromosomes will be relevant to anyone interesting in molecular evolution, especially since the female-heterogametic system of inheritance is a useful contrast to the more common male-heterogametic sex chromosomes in mammals and Drosophila. Finally, I believe that knowledge regarding basis of femaleness is of interest to a broad general community.