The unique genomic properties of sex-biased genes: Insights from avian microarray data

Background: In order to develop a framework for the analysis of sex-biased genes, we present a characterization of microarray data comparing male and female gene expression in 18 day chicken embryos for brain, gonad, and heart tissue. Results: From the 15982 significantly expressed coding regions that have been assigned to either the autosomes or the Z chromosome ( 12979 in brain, 13301 in gonad, and 12372 in heart), roughly 18% were significantly sex- biased in any one tissue, though only 4 gene targets were biased in all tissues. The gonad was the most sex- biased tissue, followed by the brain. Sex- biased autosomal genes tended to be expressed at lower levels and in fewer tissues than unbiased gene targets, and autosomal somatic sex- biased genes had more expression noise than similar unbiased genes. Sex-biased genes linked to the Z- chromosome showed reduced expression in females, but not in males, when compared to unbiased Z- linked genes, and sex- biased Z- linked genes were also expressed in fewer tissues than unbiased Z coding regions. Third position GC content, and codon usage bias showed some sex- biased effects, primarily for autosomal genes expressed in the gonad. Finally, there were several over-represented Gene Ontology terms in the sex- biased gene sets. Conclusion: On the whole, this analysis suggests that sex- biased genes have unique genomic and organismal properties that delineate them from genes that are expressed equally in males and females.