Evolution of dosage compensation on recently evolved sex chromosomes

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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.