The Mean of NNA/Ts to NNC/Gs ratios within SCs differs between (A) E. fasciculatus and (B) O. sativa. NNA/Ts: frequency of SCs having an A and a T in the third position of an amino acid, NNC/Gs: frequency of SCs having a C and a G in the third position of an amino acid. The mean is defined as the average of NNA/Ts to NNC/Gs ratios of 18 amino acids with SCs. 2EG-10EG: gene sequences arranged into between two and ten exons. The equivalent ratios in the other genomes investigated are shown in Additional file 5: Figure S4.