Sex chromosomes and the evolution of sexual dimorphism: lessons from the genome.

Females and males of many animals exhibit a striking array of sexual dimorphisms, ranging from the primary differences of the gametes and gonads to the somatic differences often seen in behavior, morphology, and physiology. These differences raise many questions regarding how such divergent phenotypes can arise from a genome that is largely shared between the sexes. Recent progress in genomics has revealed some of the actual genetic mechanisms that create separate sex-specific phenotypes, and the evidence indicates that thousands of genes across all portions of the genome contribute to male and female forms through sex-biased gene expression. Related work has begun to define the strength and influence of sex-specific evolutionary forces that shape these phenotypic dimorphisms and how they in turn affect the genome. Additionally, theory has long suggested that the evolution of sexual dimorphism is facilitated by sex chromosomes, as these are the only portions of the genome that differ between males and females. Genomic analysis indicates that there is indeed a relationship between sexual dimorphism and the sex chromosomes. However, the connection is far more complicated than current theory allows, and this may ultimately require a reexamination of the assumptions so that predictions match the accumulating empirical data.