Mapping Of Novel Hybrid Sterility Genes In Rice (Oryza Sativa L.)

Rice (O.sativa L.) is one of the most important crops, and about half of the population in the world depends on rice as primary food. The rice planting area and the yield account for about 30% and 45% of total crop growing areas and output in China, respectively. So, rice plays an important role in China. With the population increase, economic development and the farmland decrease, the food security has been increasingly paid attention. Therefore, breeding high yielding rice cultivars, especially using hybrid rice, is one of the important approaches to solve this problem. Heterosis of F1 hybrid within indica or japonica cultivars is about 10%-20% over regular cultivars. In contrast, the heterosis between indica and japonica could be 20% higher than that of intra-subspecies. However, F₁ hybrid sterility between indica and japonica is a major factor preventing application of heterosis in indica-japonica hybrids in rice production.Scientists have studied the genetics of hybrid sterility between indica and japonica over half a century. Ikehashi and Araki first proved that the sterility in F1 hybrids is caused by an allelic interaction at S5 locus on chromosome 6, where indica and japonica varieties have S5-i and S5-j, respectively, and some javanica varieties have a neutral allele, S5-n. The S5-i/S5-j genotype produces semi-sterile panicles due to the abortion of female gametes carrying S5-j. Such an abortion does not occur in S5-n/S5-i and S5-n/S5-j genotypes. The parent homozygous for S5-n is called a wide-compatibility variety (WCV). This S5-n has been incorporated into indica or japonica variety to overcome sterility problem in wide crosses and hybrid rice breeding . So far, the S5-n allele has been effective to produce fertile hybrids in a large number crosses.Along with the expansion of varietal screening, however, some hybrids showed semi-sterility despite the S5-n allele carried by one of their parents. Through varietal screening, a series of loci causing hybrid sterility, such as S7, S8, S9, S15, S16, S17 and S26(t) were detected by using isozymes and molecular markers.In the analyses of many indica-japonica hybrids, it has also been observed that there is considerable variation in the fertility level in hybrids from the same WCV crossed to different indica or japonica varieties. Thus, hybrid sterility in different varietalcombinations may be explained by allelic interaction at different lociIn some hybrids, allelic interaction at two or more independent loci caused hybrid sterility. It is therefore necessary to adopt a genome-wide approach to determine the numbers and location of the loci that may significantly influence the hybrid fertilityIn the present study, two additional loci, S29(t) and S30(t), were reported for hybrid sterility after analyzing the hybrid sterility in genome wide between Bai Mi Fen and Ketan Nangka, and Lu dao and IR36, respectively.Bai Mi Fen, a landrace of the Yunnan province in China, is a valuable source of resistant or tolerant genes to major diseases (e.g. rice stripe virus) and stresses (e.g. cold). However, the typical sterility shown in the F1 hybrids when Bai Mi Fen was crossed to Akihikari, a japonica variety and even to Ketan Nangka, ajavanica variety (Ikehashi 1987), is a barrier for the introduction of its desirable traits into the japonica and javanica varieties. A genome-wide analysis was performed for a backcrossed population of Ketan Nangka/Bei Mi Feng// Ketan Nangka using a total of 143 simple sequence repeat (SSR) markers and an EST marker to cover the entire rice linkage map. As the results, two loci were found to cause independently the hybrid sterility via female gamete abortion. Of the two loci, the locus on chromosome 4 was identified to be S9 by its chromosomal location, and the other on chromosome 2 was different from all the previously reported hybrid sterility loci, and designated as S29(t) following the hybrid sterility nomenclature. On the basis of allelic interaction which causes female gamete abortion, two alleles were found: S29(t)-kn in Keta...