When and why does sex chromosome dosage compensation evolve?

In many species, sex is determined by sex chromosomes, and the sex-specific chromosome (Y or W) stops recombining until it degenerates and carries fewer genes than its recombining counterpart (X or Z). This creates an imbalance in the dosage of most sex-linked genes between males and females. Early work in model organisms demonstrated that X chromosomes in multiple groups independently evolved regulatory mechanisms maintaining balanced expression of X-linked genes. However, recent studies have shown that these dosage compensation mechanisms are far from universal. It remains unclear why dosage compensation mechanisms evolved in some groups of organisms and not others. Two factors have led to confusion in this area: first, different authors sometimes define dosage compensation in different ways; second, dosage compensation is sometimes viewed as an all-or-nothing phenomenon, even though it may vary across cell types, developmental stages, and different classes of genes. Here, I discuss current approaches to testing for sex chromosome dosage compensation and highlight patterns in the phylogenetic distribution of dosage compensation mechanisms and possible explanations for those patterns. I conclude by outlining how the presence of dosage compensation can be tested in nearly any nonmodel organism and provide some recommendations for future studies.