The Gene Content of Mammalian and Avian Sex Chromosomes

Abstract

Sex chromosomes evolve specialised collections of genes, making them distinct from autosomes. Recent, high‐quality, genomic sequencing of sex chromosomes allows us to trace the evolutionary history of the mammalian X and Y chromosomes and avian Z and W chromosomes, and examine how evolution has remodelled their gene content. The XY and ZW sex chromosomes arose from ordinary pairs of autosomes. Differentiation into specialised sex chromosomes initiated with the degeneration of gene content on the Y and W chromosomes and the preservation of gene content on the X and Z chromosomes. As these degeneration and preservation processes unfolded, the sex chromosomes also acquired new genes, many of which are located in large, nearly identical, segmental duplications called amplicons. Ampliconic sequences, independently evolving across multiple lineages, are a major driving force in sex chromosome evolution. Key Concepts Mammalian and avian sex chromosomes evolved independently from autosomes. X and Z chromosomes retain the majority of the ancestral autosomal genes. X chromosome ancestral autosomal genes are highly conserved across mammals. The gene content of Y and W chromosomes has degenerated. Ampliconic sequences have independently evolved on X, Y, Z and W chromosomes.