Genomic evidence for a large-Z effect.

The 'large-X effect' suggests that sex chromosomes play a disproportionate role in adaptive evolution. Theoretical work indicates that this effect may be most pronounced in genetic systems with female heterogamety under both good-genes and Fisher's runaway models of sexual selection (males ZZ, females ZW). Here, I use a comparative genomic approach (alignments of several thousands of chicken-zebra finch-human-mouse-opossum orthologues) to show that avian Z-linked genes are highly overrepresented among those bird-mammalian orthologues that show evidence of accelerated rate of functional evolution in birds relative to mammals; the data suggest a twofold excess of such genes on the Z chromosome. A reciprocal analysis of genes accelerated in mammals found no evidence for an excess of X-linkage. This would be compatible with theoretical expectations for differential selection on sex-linked genes under male and female heterogamety, although the power in this case was not sufficient to statistically show that 'large-Z' was more pronounced than 'large-X'. Accelerated Z-linked genes include a variety of functional categories and are characterized by higher non-synonymous to synonymous substitution rate ratios than both accelerated autosomal and non-accelerated genes. This points at a genomic 'large-Z effect', which is widespread and of general significance for adaptive divergence in birds.