Why does dosage compensation differ between XY and ZW taxa?

Recent studies have demonstrated an extensive male bias in the gene expression on Z chromosomes in species with a ZW sex determination (e.g. birds), suggesting that ZW females have incomplete dosage compensation. We propose that the extensive male bias on Z chromosomes is caused by the functional properties of male-adapted genes, which make them unsuitable for high expression in females, and that ZW females are dosage-compensated to a point where they have achieved enough compensation to maintain the integrity of critical networks. In ZW systems, Z chromosomes spend two-thirds of their evolutionary time in males, and pronounced sexual selection in males and the male mutation bias can interact and affect levels of sexual antagonism, causing more extensive male bias in gene expression on Z-linked genes. These patterns and processes contrast with those acting on genes on the X chromosome in XY species, and might help explain the discrepancies in the degree of sex-biased gene expression and dosage compensation in XY and ZW systems. 2009 Elsevier Ltd. All rights reserved.