Changes in gene expression during sex chromosome evolution in the dioecious plant Silene latifolia

我们建议研究植物宽叶木性染色体上的基因表达水平。这种植物是少数不两性的植物物种之一，但有不同的性别个体，以及与哺乳动物(包括人类)一样的X/Y性染色体性别决定系统。众所周知，Y染色体(男性决定染色体)是从与其他染色体相似的祖先染色体进化而来的，雌性哺乳动物的X染色体与其他染色体仍然没有太大的区别，尽管它们有一些独特的特征，可能反映了它们作为性染色体的进化。然而，Y染色体在两种哺乳动物中都非常奇怪，在另一种研究得很好的系统中，果蝇，果蝇。在这两种情况下，Y染色体只携带很少的基因，而且只有一小部分基因存在于X染色体上(在果蝇中，X和Y染色体之间似乎没有共享的基因，在哺乳动物中有少数这样的基因)；在这两种情况下，Y染色体都获得了X染色体上没有的基因。这些奇怪的遗传特性是如何进化出来的，这是一个有趣的问题。关于Y染色体从大多数基因座与X染色体共享的初始状态开始的“遗传侵蚀”(或“退化”)过程的理论认为，Y染色体上的基因失去了原来的功能，然后可以被删除，这是因为X和Y染色体不会像其他染色体那样交叉和进行基因重组。没有基因重组的基因组区域的种群遗传模型预测，有利基因不太可能传播，而自然选择将无法抵抗不利基因的传播。为了研究这种遗传退化过程，我们需要位于发生退化的物种的性染色体上的基因。因为哺乳动物和果蝇的性染色体很久以前就进化了，它们的Y染色体已经丢失了X染色体上的大部分基因，所以它们不太适合进行这样的研究。已知一些植物物种的性染色体进化时间比这些动物晚得多，这对研究Y染色体遗传侵蚀很有意义。我们建议研究植物宽叶银莲性染色体，即白色露营植物。它的性染色体从遗传学早期就开始进行细胞学研究，近年来已经对性染色体上的一些基因进行了测序。我们的项目是通过测试几个基因的Y染色体副本是否比相同基因的X连锁副本的表达水平更低，来直接测试遗传退化。这现在是可能的，因为我们已经开发出一种直接的方法来识别与性相关的基因(允许我们在性染色体上发现一些新的基因)；因此，现在有足够的基因座来了解Y副本是否具有低表达水平，并且很容易在性染色体上找到更多的基因。该项目包括测试，以显示所发现的任何差异是否源于低Y表达，而不是X拷贝表达增加的可能性较小，但也很有趣，因为该物种与其两性近亲--膀胱坎皮安--普通硫杆菌分离。表达实验之所以可能，是因为X和Y拷贝足够相似，可以在实验中同时扩增两者的短序列，但它们的序列差异足够大，可以测量两个不同等位基因的表达。

We propose studies of expression levels of genes on the sex chromosomes of the plant Silene latifolia. This plant is one of the minority of plant species that is not hermaphroditic, but has separate sexed individuals, and an X/Y sex chromosomal sex-determination system like that in mammals, including humans. It is well known that the Y (male-determining) chromosomes evolved from an ancestral chromosome that was similar to other chromosomes, and the X chromosomes in female mammals are still not very different from other chromosomes, though they have some distinctive features that probably reflect their evolution as sex chromosomes. The Y chromosomes, however, are extremely strange in both mammals and in the other very well studied system, the fruitfly, Drosophila. In both cases, the Y chromosomes carry only very few genes, and only a small proportion of them are present on the X (in Drosophila melanogaster, no genes appear to be shared between the X and Y chromosomes, and in mammals there is a handful of such genes); in both cases the Y has acquired genes not present on the X. How these strange genetic properties evolved is an interesting question. Theories for the process of 'genetic erosion' (or 'degeneration') of Y chromosomes from an initial state in which most loci were shared with the X chromosome, suggest that genes on Y chromosomes lose their former functions and can then be deleted, and that this occurs because the X and Y chromosomes do not cross-over and undergo genetic recombination like other chromosomes. Population genetic models of regions of genome without genetic recombination predict that advantageous genes are less likely to be able to spread, and that natural selection will be unable to resist spread of disadvantageous genes. To study this genetic degeneration process, we need genes that are located on the sex chromosomes of a species in which degeneration is occurring. Because mammalian and fruitfly sex chromosomes evolved long ago, their Y have already lost most genes present on the X, so they are not well suited for such studies. Some plant species are known to have evolved sex chromosomes much more recently than these animals, and these are of interest for studying Y chromosome genetic erosion. We propose to work on the sex chromosomes of the plant Silene latifolia, the white campion. Its sex chromosomes have been studied cytologically since the early days of genetics, and in recent years a few genes on the sex chromosomes have been sequenced. Our project is to test in a direct manner for genetic degeneration by testing whether Y chromosomal copies of several genes are expressed at lower levels than X-linked copies of the same genes. This is now possible because we have developed a straightforward approach to identifying sex-linked genes (allowing us to discover some new genes on the sex chromosomes); thus there are now enough loci to gain a general picture of whether Y copies have low expression levels, and more genes on the sex chromosomes can readily be found. The project includes tests to show whether any differences found are due to low Y expression , versus the less likely, but also interesting, possibility of an increase in expression of X copies since the species separated from its hermaphrodite relative, the bladder campion, S. vulgaris. The expression experiments are possible because the X and Y copies are similar enough that short sequences of both can be amplified simultaneously in experiments, yet their sequences differ enough that expression of each of the two different alleles can be measured.