Sex chromosomes in evolution and development

In many species, specialized "sex chromosomes", such as the X and Y of mammals, or Z and W of birds (in this case females are ZW, males ZZ), carry the gene(s) that lead to the development of a male or a female. Although they originate from normal pairs of chromosomes, sex chromosomes often become very different from each other over evolutionary time: Y and W chromosomes degenerate (i.e. they lose most of their genes); to make up for these missing genes, the expression of genes on X and Z chromosomes can be modified, ensuring that males and females both have balanced expression levels. This phenomenon is called "dosage compensation", and is well described in mammals and insects. Many aspects of the interplay between sex chromosome evolution, the acquisition of dosage compensation, and the life cycle of the organism are not fully understood. Our work will fill several of these knowledge gaps. First, we know that new sex chromosomes appear all the time, but how new sex-determining genes on these chromosomes originate, and how they are incorporated into embryonic development, a tightly regulated process, is not fully understood. We will therefore first investigate how a complex signaling network was recruited for sex determination in Drosophila. Even in model organisms with well described mechanisms of dosage compensation, there is an unresolved paradox: dosage compensation can only be established after the embryo has started expressing its own genes and figured out its own sex, leading to dosage imbalances in very early development. How such imbalances are handled at such a tightly regulated stage is almost entirely unknown, and will be the second focus of our work. Paradoxical as these imbalances in early embryos may be, they pale in comparison to what is happening in chicken, where only the copy of the genome provided by the mother seems to be used by young embryos to produce gene expression. Since female embryos only inherit a degenerated W from their mothers (but no large Z chromosome), it is unclear how such a partial activation is possible, and this will also be investigated here. Finally, we will investigate why X chromosomes are packed away and cannot produce gene expression during the sperm production of many species. In particular, we will take advantage of new single-cell technologies to test if a similar "inactivation" is found in species with ZW sex chromosomes, something that has so far not been extensively studied.