Figure 1.—

Divergence as a function of expression in S. cerevisiae. For each of 764 genes (the members of qualified gene pairs from the ancient duplication; see materials and methods ), the closest homolog is found. The modified Jukes-Cantor divergence score for the pair, if defined (materials and methods), is plotted on the y-axis, on the basis of the ClustalW pairwise alignment of the genes. The CAI of the chosen gene is plotted on the x-axis. CAI is a surrogate for the level of mRNA produced. Note that the divergence of each gene pair (g₁, g₂) is plotted twice, once against the CAI of g₁ and once against the CAI of g₂. There is a strong negative correlation between divergence and CAI (i.e., level of expression), with correlation coefficient r = −0.72 (P < 10⁻¹⁶ for the null hypothesis that r = 0).