Evolution of gene function on the X chromosome versus the autosomes.

Sex chromosomes have arisen from autosomes many times over the course of evolution. This process generates chromosomal heteromorphy between the sexes, which has important implications for the evolution of coding and noncoding sequences on the sex chromosomes versus the autosomes. The formation of sex chromosomes from autosomes involves a reduction in gene dosage, which can modify properties of selection pressure on sex-linked genes. This transition also generates differences in the effective population size and dominance characteristics of novel mutations on the sex chromosome versus the autosomes. All of these changes may affect both patterns of in situ gene evolution and the rates of interchromosomal gene duplication and movement. Here we present a synopsis of the current understanding of the origin of sex chromosomes, theoretical context for differences in rates and patterns of molecular evolution on the X chromosome versus the autosomes, as well as a summary of empirical molecular evolutionary data from Drosophila and mammalian genomes.