A triallelic system of S5 is a major regulator of the reproductive barrier and compatibility of indica–japonica hybrids in rice

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Abstract
Hybrid sterility is a major form of postzygotic reproductive isolation. Although reproductive isolation has been a key issue in evolutionary biology for many decades in a wide range of organisms, only very recently a few genes for reproductive isolation were identified. The Asian cultivated rice ( Oryza sativa L.) is divided into two subspecies, indica and japonica . Hybrids between indica and japonica varieties are usually highly sterile. A special group of rice germplasm, referred to as wide-compatibility varieties, is able to produce highly fertile hybrids when crossed to both indica and japonica . In this study, we cloned S5 , a major locus for indica–japonica hybrid sterility and wide compatibility, using a map-based cloning approach. We show that S5 encodes an aspartic protease conditioning embryo-sac fertility. The indica ( S5 -i) and japonica ( S5 -j) alleles differ by two nucleotides. The wide compatibility gene ( S5 -n) has a large deletion in the N terminus of the predicted S5 protein, causing subcellular mislocalization of the protein, and thus is presumably nonfunctional. This triallelic system has a profound implication in the evolution and artificial breeding of cultivated rice. Genetic differentiation between indica and japonica would have been enforced because of the reproductive barrier caused by S5 -i and S5 -j, and species coherence would have been maintained by gene flow enabled by the wide compatibility gene.