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

我们提出的植物宽叶蝇子草的性染色体上的基因的表达水平的研究。这种植物是少数不是雌雄同体的植物物种之一，但有单独的性别个体，以及像哺乳动物（包括人类）一样的X/Y性染色体性别决定系统。众所周知，Y（雄性决定）染色体是从一个与其他染色体相似的祖先染色体进化而来的，而雌性哺乳动物的X染色体与其他染色体仍然没有很大的不同，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染色体在哺乳动物和另一个研究得很好的系统中都是非常奇怪的，果蝇。在这两种情况下，Y染色体只携带很少的基因，只有一小部分基因存在于X染色体上（在黑腹果蝇中，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染色体上不存在的基因。这些奇怪的遗传特性是如何进化的是一个有趣的问题。Y染色体从最初状态的“遗传侵蚀”（或“退化”）过程的理论表明，Y染色体上的基因失去了它们以前的功能，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遗传重组。没有基因重组的基因组区域的群体遗传模型预测，有利基因不太可能传播，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，我们需要位于退化发生的物种的性染色体上的基因。由于哺乳动物和果蝇的性染色体在很久以前就进化了，它们的Y染色体已经失去了X染色体上的大多数基因，因此它们不太适合进行此类研究。已知一些植物物种比这些动物进化出性染色体的时间要晚得多，这对研究Y染色体遗传侵蚀很有意义。我们打算对植物宽叶蝇子草（白色坎皮翁）的性染色体进行研究。从遗传学的早期开始，人们就对它的性染色体进行了细胞学研究，近年来，性染色体上的一些基因已经被测序。我们的项目是通过测试几个基因的Y染色体拷贝的表达水平是否低于相同基因的X连锁拷贝，以直接的方式测试遗传变性。现在这是可能的，因为我们已经开发了一种直接的方法来识别性连锁基因（让我们发现性染色体上的一些新基因）;因此，现在有足够的基因座来获得Y拷贝是否具有低表达水平的总体图像，并且可以容易地找到性染色体上的更多基因。该项目包括测试以显示所发现的任何差异是否是由于Y表达低，相对于不太可能，但也有趣的是，X拷贝表达增加的可能性，因为该物种从其雌雄同体的亲戚，膀胱坎皮翁，S。普通的表达实验是可能的，因为X和Y拷贝足够相似，使得两者的短序列可以在实验中同时扩增，但它们的序列足够不同，使得可以测量两个不同等位基因中的每一个的表达。

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.