Dosage constraints on gene evolution. In Panel A, the correlation between expression profiles of wheat and barley genes linked by reciprocal Blast hits in the homologous clusters is plotted against the overall mean expression level for these gene pairs. The profiles exhibit higher correlation if the actual expression levels themselves are high, suggesting that gene function tends to be more conserved for highly expressed genes. In Panel B, the mean expression is plotted against sequence similarity (i.e. E-values of the Blast hits), indicating that there is an analogous correlation between the level of expression and sequence similarity.