Mammalian Sex Chromosome Evolution

Abstract

Abstract Although they are highly dissimilar in sequence content in extant organisms, the mammalian sex chromosomes (i.e. the X and Y) evolved from a pair of autosomes. Differentiation of the sex chromosomes during evolution was initiated by the suppression of X–Y recombination, likely through changes in genomic structure. This was followed by the degeneration of genes on the Y chromosome, with specific instances of gene specialisation on the nonrecombining portion of the Y. Findings of evolutionary ‘strata’ on the sex chromosomes of mammals suggest that multiple events have given rise to the current arrangement of genes on the X and Y chromosomes of mammals. However, new studies utilising more divergent genomic data are challenging conclusions regarding the number and frequency of events that led to the current arrangement of mammalian sex chromosomes, and future studies with complete genomic sequences of marsupials will continue to further our understanding of the evolution of these critical chromosomes. Key Concepts: Mammalian sex chromosomes arose from a pair of autosomes. Genomic rearrangements repressed recombination in a stepwise fashion on these chromosomes. Genes on these chromosomes diverged due to a combination of forces, including sexual antagonism and removal of deleterious mutations. Chromosomal studies outside placental mammals continue to contribute to our understanding of mammalian sex chromosome evolution. Groups of genes on sex chromosomes with similar evolutionary origins are called gene ‘strata’ and exist on both the X and Y chromosomes in the nonrecombining regions.