Understanding the processes of the origination and biological mechanisms of reproductive isolation has been of fundamental importance in evolutionary biology.Hybrid sterility between indica and japonica subspecies of Asian cultivated rice(Oryza sativa L.)is a well-known example of postzygotic reproductive isolation in plants.The killer-protector system of S5 in rice,composed of three adjacent genes,regulates female sterility of indica-japonica hybrids,which contributes significantly to reproductive isolation between indica and japonica subspecies.