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Abstract

In mammals, X-chromosome inactivation (XCI) re-establishes the dosage balance between male and female gene expression levels. However, up to a third of the X-linked genes escape this phenomenon with varying degrees of consistency across tissues, cell lines, or individuals. Here, I take advantage of a new extensive developmental dataset for several organs and species to explore the developmental dimension of sex chromosomes expression levels. I find that only a small fraction of genes escaping XCI show a consistent female overexpression across development, organs, and species. The consistently sex-biased genes are almost exclusively either directly involved in the establishment of XCI, or are protein-coding genes with a broadly expressed Y homolog. The conservation of the sex-bias of these genes across species suggests that they might be responsible for the evolution of escape from XCI. I also present and test a model of allelic contribution to the total expression levels of X-linked genes using marsupial bulk tissue RNA sequencing data. Finally, I describe my contribution to a study of sex-biased micro-RNAs in mammals.