Evolution of dosage compensation on recently evolved sex chromosomes

在大多数物种中，个体的发育是雄性还是雌性取决于性染色体的存在；例如，在人类中，x -个体发育为雄性，而x -个体发育为雌性。在性染色体独立进化的许多其他生物中也存在类似的情况。尽管性染色体有多个独立的起源，但在不同的生物体中，性染色体的特性却相当相似，这表明形成性染色体的进化力量具有普遍性。特别是，y染色体往往失去功能基因，变得遗传退化，而x染色体上的基因保持功能。例如，人类的y染色体只包含几十个功能基因，而x染色体上有大约1000个基因。挑战在于理解形成性染色体的进化力量，目前这是正在进行的研究的一个非常热门的话题。从y染色体上丢失的基因在男性中只留下一个有性连锁基因的功能拷贝，而在女性中则有两个基因拷贝（因为她们有两条x染色体）。这造成了两性之间的不平衡，对男性来说可能是有害的，因为性别相关基因产生的产物不足。为了解决这个问题，雄性果蝇（Drosophila melanogaster）通过增加x连锁基因的产量来弥补雄性x连锁基因第二拷贝的缺失。在其他生物体中也发现了这种“剂量补偿”，但它们实现两性平衡的方式各不相同。例如，哺乳动物在雄性和雌性中增加x连锁基因的表达，然后在雌性中关闭一条x染色体。这种剂量补偿系统是如何进化的仍然知之甚少，而且“古老”的性染色体，如哺乳动物和黑腹果蝇的性染色体，几乎不适合研究剂量补偿的进化。本项目将利用植物银叶性染色体的最新进化，研究剂量补偿系统进化的早期步骤。Silene属的大多数物种是雌雄同体的，而一些物种，包括Silene latifolia，在相对较近的时间里进化出了不同的性别，这使得我们可以比较不同交配系统下的近亲物种。重要的是，大叶树的性别是由性染色体决定的，这些性染色体从无到有进化了几百万年。已经证明，该物种的y染色体已经开始退化，新生剂量补偿存在于x染色体上。近年来，女花性染色体的起源和同属物种的存在，代表了没有性染色体的祖先状态，提供了一个独特的机会来揭示在其进化的早期阶段形成性染色体的进化力量和过程。

In most species, the development of an individual as a male or a female depends on the presence of sex chromosomes; for example, in humans, XY-individuals become males and XX-individuals develop as females. Similar situation exists in many other organisms where sex chromosomes evolved independently. Despite multiple independent origins, the properties of sex chromosomes are rather similar in different organisms, indicating the generality of evolutionary forces shaping sex chromosomes. In particular, Y-chromosome tends to lose functional genes and become genetically degenerate, while genes on the X-chromosome remain functional. For example, human Y-chromosome contains only a few dozen functional genes, which contrasts with about a thousand genes on the X-chromosome. The challenge is to understand the evolutionary forces shaping sex chromosomes and currently this is a very hot topic of on-going research.The loss of genes from Y-chromosome leaves only one functional copy of a sex-linked gene in males, while two copies of the gene are present in females (because they have two X-chromosomes). This creates disbalance between sexes and is likely to be detrimental in males, where not enough product is produced from sex-linked genes. To resolve this problem, fruit fly (Drosophila melanogaster) males increase yield from X-linked genes to compensate for lack of second copy of X-linked genes in males. Such 'dosage compensation' has also been discovered in other organisms, but the way they achieve the balance between sexes varies. For example, mammals increase expression of X-linked genes in both males and females and then switch off one of the X-chromosomes in females. How such dosage compensation systems evolve remains poorly understood, and 'old' sex chromosomes, such as those in mammals and Drosophila melanogaster, are hardly suitable to study evolution of dosage compensation.This project will take advantage of the recent evolution of sex chromosomes in plant Silene latifolia to study the early steps in evolution of dosage compensation system. Most species in genus Silene are hermaphroditic, while a few species, including Silene latifolia, evolved separate sexes relatively recently, allowing us to compare closely related species with different mating systems. Importantly, sex in S. latifolia is determined by sex chromosomes that evolved from scratch within only few million years. It has been demonstrated that Y-chromosome in this species has begun degenerating and nascent dosage compensations is present on the X-chromosome. The recent origin of S. latifolia sex chromosomes and the presence of congeneric species, representing the ancestral state without sex chromosomes, provide a unique opportunity to unravel the evolutionary forces and processes shaping the sex chromosomes at the early stages of their evolution.