Several lines of evidence suggest that the X chromosome of various animal species has an unusual complement of genes with sex-biased or sex-specific expression. However, the study of the X chromosome gene content in different organisms provided conflicting results. The most striking contrast concerns the male-biased genes, which were reported to be almost depleted from the X chromosome in Drosophila but overrepresented on the X chromosome in mammals. To elucidate the reason for these discrepancies, we analysed the gene content of the Z chromosome in chicken. Our analysis of the publicly available expressed sequence tags (EST) data and genome draft sequence revealed a significant underrepresentation of ovary-specific genes on the chicken Z chromosome. For the brain-expressed genes, we found a significant enrichment of male-biased genes but an indication of underrepresentation of female-biased genes on the Z chromosome. This is the first report on the nonrandom gene content in a homogametic sex chromosome of a species with heterogametic female individuals. Further comparison of gene contents of the independently evolved X and Z sex chromosomes may offer new insight into the evolutionary processes leading to the nonrandom genomic distribution of sex-biased and sex-specific genes.

Institute of Molecular Genetics Academy of Sciences of Czech Republic and Center for Applied Genomics Vídenská 1083 CZ 142 20 Prague 4 Czech Republic

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The chicken Z chromosome is enriched for genes with preferential expression in ovarian somatic cells