POSITIVE DARWINIAN SELECTION AFTER GENE DUPLICATION IN PRIMATE RIBONUCLEASE GENES

Evolutionary mechanisms of origins of new gene function have been a su bject of long-standing debate, Here we report a convincing case in whi ch positive Darwinian selection operated at the molecular level during the evolution of novel function by gene duplication, The genes for eo sinophil cationic protein (ECP) and eosinophil-derived neurotoxin (EDN ) in primates belong to the ribonuclease gene family, and the ECP gene , whose product has an anti-pathogen function not displayed by EDN, wa s generated by duplication of the EDN gene about 31 million years ago, Using inferred nucleotide sequences of ancestral organisms, we showed that the rate of nonsynonymous nucleotide substitution was significan tly higher than that of synonymous substitution for the ECP gene, This strongly suggests that positive Darwinian selection operated in the e arly stage of evolution of the ECP gene, It was also found that the nu mber of arginine residues increased substantially in a short period of evolutionary time after gene duplication, and these amino acid change s probably produced the novel anti-pathogen function of ECP.