Sex chromosome dosage compensation: definitely not for everyone

• Males and females experience different gene dosage for sex-linked genes, which can cause differences in expression.
• In some species, sex chromosome dosage compensation has evolved to balance out these these differences.
• In other species, dosage compensation is incomplete, and expression for many genes differs between females and males.
• Complete sex chromosome dosage compensation is limited to X chromosomes and is incomplete on Z chromosomes.

Sex chromosomes often entail gene dose differences between the sexes, which if not compensated for, lead to differences between males and females in the expression of sex-linked genes. Recent work has shown that different organisms respond to sex chromosome dose in a variety of ways, ranging from complete sex chromosome dosage compensation in some species to active compensation of only a minority genes in other organisms. Although we still do not understand the implications of the diversity in sex chromosome dosage compensation, its realization has created exciting new opportunities to study the evolution, mechanism, and consequences of gene regulation. However, confusion remains as to what sorts of genes are likely to be dosage compensated, how dosage compensation evolves, and why complete dosage compensation appears to be limited to male heterogametic species. In this review, I survey the status of dosage compensation to answer these questions and identify current controversies in this fast-moving field.