Asymmetry in the Strength of Sexual Antagonism in XY vs. ZW Systems – Evidence from Whole-Genome Expression

摘要

Sexual antagonism refers to selective pressure that operates on the two sexes differently, especially when a mutation is beneficial in one sex but deleterious in the other. Sexual antagonism has interest-ing consequences in species with chromosomal sex-determination. Prior studies have established that gene expression on the X chromosome tends to be female-biased. For the same reason, the op-posite for the Z chromosome may also be true. Was the force driving the bias mainly sexual selec-tion or does natural selection play a key role? Is the strength of sexual antagonism the same in the XY vs ZW systems? If sexual selection is the main driving force, one may expect male-bias on Z to be stronger than female bias on X in species where sexual selection on males is stronger than on females. The asymmetry should be testable and observable in both ZZ/ZW and XX/XY system. In this study, we analyze sex-biased expression profiles for somatic tissues and gonad in Drosophila melanogaster, mice, and chicken embryo and adults. We observe that X chromosome is sex-biased in Drosophila and mammals, but mostly gonad-based. In somatic tissue, the level of sex-bias in chicken with homogametic males is dramatically different from that in fly and mammal, the system with heterogametic males. The level of sex bias is weak overall in somatic tissue of mouse and Drosophila. In birds, the expression level of Z-linked genes in soma is shifted significantly to be male-biased relative to autosome. Our result suggests that sexual antagonism is asymmetric in males than in females.