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

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 indicajaponica 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.subspecies of rice ͉ hybrid sterility ͉ wide compatibility ͉ aspartic protease

Chromosomal Location of HWA1 and HWA2, Complementary Hybrid Weakness Genes in Rice

“…Regarding the reproductive barrier in rice, causal genes were cloned in an allelic interaction case: heterozygotes of Indica allele S5-i and Japonica allele S5-j at the S5 locus induce embryo-sac semi-sterility caused by partial abortion of female gametes carrying the Japonica alleles. Chen et al (2008) sequence level. Chen et al (2008) did not explain the molecular mechanism caused by the heterozygous form at the S5 locus.…”

“…Chen et al (2008) sequence level. Chen et al (2008) did not explain the molecular mechanism caused by the heterozygous form at the S5 locus. In another case, causal genes of reproductive barrier in rice, which had been thought to be allelic before, proved to be a complex of two very tightly linked genes.…”

“…Many postzygotic reproductive barrier forms have been reported in rice (Oryza sativa), such as hybrid weakness (e.g., Oka 1957), hybrid pollen sterility (e.g., Long et al 2008), and hybrid sterility causing female gamete abortion (e.g., Chen et al 2008). Among them, hybrid weakness is definable as weak growth occurring in F 1 hybrids derived from crosses between two normal strains.…”

Prediction of Hybrid Performance in Crop Plants: Molecular and Recent Approaches

“…Chen et al, (2008) cloned S5, a major locus for indica japonica hybrid sterility and wide compatibility, using a map-based cloning approach and found that S5 encodes an aspartic protease conditioning embryo-sac fertility and a discontinuous 136-bp deletion. The same aspartic protease was reported in wide-compatible varieties carrying the neutral allele at S5 locus compared to the indica and japonica alleles using S5 multiplex marker system.…”

A multiplex PCR system for detection of wide compatibility allele S5-n and erect panicle allele dep1 in rice

“…Plants with the S5-n/S5-j (or S5-i) genotype are fully fertile, while plants with the S5-i/S5-j genotype have low fertility. Chen et al (2008) cloned the S5 gene from the wide-compatibility variety 02428 using a map-based cloning approach and characterized its function. The S5 of indica and japonica differ by two nucleotides, resulting in an amino acid difference and thus leading to hybrid F 1 sterility.…”