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Evolution of sex chromosomes and sex-biased gene expression

Sex chromosomes have evolved from non-sex-determining chromosomes (autosomes) many times throughout the tree of life. Ever since their discovery, these chromosomes have captivated researchers because of their obvious involvement in fundamental aspects of an organism’s life, such as sex determination, sexual reproduction and sexual conflicts. Despite this long-lasting fascination, their biology remains unclear in several central aspects, in particular regarding their almost ubiquitous evolution of recombination cessation and impoverished gene content, and the multifaceted consequences thereof. This lack of detailed knowledge about sex chromosome evolution compromises our understanding of fundamental biological questions (e.g. the evolution of sexual conflicts) as well as more practical ones (e.g. about sex-linked genetic diseases).

Our research focuses mainly on Sylvioidea passerine birds where we have recently detected a new sex chromosome, a neo-sex chromosome, which has been formed by a fusion between an autosome and the ancestral sex chromosomes. This neo-sex chromosome is evolutionarily very interesting and promising for research: First, the female-specific neo-W chromosome is in an intermediate stage of degeneration. This is an ideal situation for research on sex chromosomes, because the processes we wish to study have had some time to leave detectable genomic footprints, but have not yet removed the material we want to investigate. Second, the neo-sex chromosome is homologous to the sex chromosomes of mammals, which makes it possible – for the first time – to study mammalian X gene homologs in a novel, avian Z-linked environment where females are the heterogametic sex (ZW), and thus to draw direct parallels over the avian ZW–mammalian XY boundary.

Evolution of sex chromosomes in songbirds

We are now exploring this remarkable neo-sex chromosome system to understand the evolution of sex chromosomes in terms of recombination cessation and gene content. This includes

  1. assembling the genome of one of our main Sylvioidea study species, the great reed warbler
  2. measuring gene expression in males and females to evaluate the evolution of sex-biased expression and dosage compensation
  3. conduct comparative genomics within Sylvioidea and between Sylvioidea and mammals.
  4. Finally, we use our unique 30–year ecological dataset and rich collection of DNA samples of the great reed warbler to test the widely accepted but poorly supported sexual antagonism hypothesis suggesting that sexually antagonistic mutations accumulate on the sex chromosomes and drive their evolution.

We have also started to study a few other interesting sex chromosome systems, including in plants, damselflies and lizards

Summary of main research objectives:

  • To understand sex chromosome evolution in terms of recombination cessation, degeneration and gene content.
  • To understand the evolution of sex-biased gene expression and dosage compensation, and whether incomplete dosage compensation disrupts critical gene co-expression networks.
  • To evaluate possible convergent evolution of the same sex-linked genes in birds and mammals.
  • To understand the interplay between sex chromosomes and sexual conflicts by studying whether sexually antagonistic mutations accumulate on sex chromosomes, and whether sexual conflicts drive sex chromosome evolution.
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Great Reed Warbler
Great Reed Warbler