Faster evolution of Z-linked duplicate genes in chicken

It has been shown that duplicate genes on the X chromosome evolve much faster than duplicate genes on autosomes in Drosophila melanogaster. However, whether this phenomenon is general and can be applied to other species is not known. Here we examined this issue in chicken that have heterogametic females (females have ZW sex chromosome). We compared sequence divergence of duplicate genes on the Z chromosome with those on autosomes. We found that duplications on the Z chromosome indeed evolved faster than those on autosomes and show distinct patterns of molecular evolution from autosomal duplications. Examination of the expression of duplicate genes revealed an enrichment of duplications on the Z chromosome having male-biased expression and an enrichment of duplications on the autosomes having female-biased expression. These results suggest an evolutionary trend of the recruitment of duplicate genes towards reproduction-specific function. The faster evolution of duplications on Z than on the autosomes is most likely contributed by the selective forces driving the fixation of adaptive mutations on Z. Therefore, the common phenomena observed in both flies and chicken suggest that duplicate genes on sex chromosomes have distinct dynamics and are more influenced by natural selection than autosomal duplications, regardless of the kind of sex determination systems.