The emergence of new genes on the young therian X

Recent studies have revealed that our sex chromosomes differentiated relatively recently from ancestral autosomes in the common ancestor of placental and marsupial mammals (therians). Here, we show that the therian X started to accumulate new retroduplicate genes with overall sex-biased expression upon therian sex chromosome differentiation. This process reached its peak within the first ∼90 million years of sex chromosome evolution and then leveled off. Taken together, our observations suggest that the major sex-related functional remodeling of the X was completed relatively soon after the origination of therian sex chromosomes.