Nonrandom distribution of genes with sex-biased expression in the chicken genome

Evolutionary theory predicts that sexually antagonistic genes should show a nonrandom genomic distribution with sex chromosomes usually being enriched for such genes. However, empirical observations from model organisms (Drosophila melanogaster, Caenorhabditis elegans, mammals) on the genomic location of genes with sex-biased expression have provided conflicting data and are not easily explained by a unified framework based on standard models of the evolution of sexually antagonistic genes. Previous studies have been confined to organisms with male heterogamety, meaning that effects related to homo- or heterozygosity of sex chromosomes cannot be separated from effects related to sex-specific characteristics. We therefore studied the genomic distribution of genes with sex-biased expression in the chicken, that is, in an organism with female heterogamety (males ZZ, females ZW). From the abundance of transcripts in expressed sequence tag libraries, we found an underrepresentation of female-specific genes (germ line and somatic tissue) and an overrepresentation of male-specific genes (somatic) on the Z chromosome. This is consistent with theoretical predictions only if mutations beneficial to one sex generally tend to be at least partly dominant (h > 0.5). We also note that sexual selection for a male-biased trait is facilitated by Z-linkage, because sons in organisms with female heterogamety will always inherit a Z chromosome from their fathers.