FIG. 1.—

Plots of d_N/d_S for rodent and primate seminal fluid protein genes. (A) Seminal fluid protein ESTs identified in the MPED for mouse versus rat. (B) Genes encoding seminal fluid proteins identified by mass spectrometry in human versus chimpanzee (from Clark and Swanson [2005]). The diagonal represents neutral evolution, a d_N/d_S ratio of one. Most genes are subject to purifying selection and fall below the diagonal, whereas several genes fall above or near the line suggesting positive selection. Comparison of the two plots shows elevated d_N/d_S ratios in seminal fluid genes of both taxonomic groups.