The Mechanisms Of Inter-subspecies Hybrid Semi-sterility And Fine Mapping Of A New Hybrid Embryo Sac Sterility Gene

Rice plays an important role in food security in China. Using the heterosis ofinter-subspecies is one of the important approaches to increase the rice yield.However, the semi-sterility has been a major barrier for the utilization of strongheterosis between indica and japonica. The discovery of wide compatibility genes(WCGs) has brought breeders hope for breaking the fertility barrier. The widecompatibility gene S5-n has been incorporated into breeding lines to utilize the strongheterosis of indica-japonica hybrids in hybrid rice breeding. However, with thebroadening of parents used in rice breeding, some hybrids still showed semi-sterilitydespite the present of S5-n allele in one of their parents. In order to take the fulladvantage of the strong heterosis of inter-subspecific hybrids, it is necessary toaccumulate the wide compatibility genes found in useful genetic backgrounds toovercome various sterilities in inter-subspecific hybrids. The genetic study of hybridsterility led to the finding of a neutral allele or wide compatibility allele.Tuanguzao, a landrace from Yunnan province, China, is a valuable source ofmultiple resistances to major diseases (e.g. rice stripe virus) and stresses (e.g. cold).Ketan Nangka is a javanica variety carrying wide compatibility genes at loci S5 andS15 (Wan and Ikehashi, 1996b). However, the typical partial sterility shown in the F₁hybrids between Ketan Nangka and Tuanguzao is a genetic barrier for the introductionof desirable genes in Tuanguzao. In this study, we (1)reveal the cytologicalmechanisms of the semi-sterility in the F₁ hybrid between Tuanguzao and KetanNangka and identify the neutral alleles that can overcome the sterility; (2) map thegenes controlling hybrid semi-sterility through a genome-wide analysis; and (3)conduct fine mapping of a new sterility gene detected in the study. The results are asfollow.1, The partial abortion of embryo sac was the major cause for the semi-sterility in theF₁ hybrid between Tuanguzao and Ketan Nangka. Several types of abnormalities wereobserved during consecutive stages of embryo sac formation and development, the most often abnormalities were observed at functional macrospore stage, degeneratedfunctional macrospore prevent the normal development of embryo sacs.2, Genome-wide analysis of the linkage map constructed from the backcrosspopulation of Tuanguzao/Ketan Nangka//Ketan Nangka identified two indepedent loci,qSS2 and qSS5, responsible for the hybrid sterility located on chromosomes 2 and 5,which explained 18.6% and 20.1% of phenotypic variance, respectively, qSS5 mappedto the previously reported sterility gene S31(t), while qSS2, a new hybrid sterility gene,was tentatively designated as S32(t). The hybrid sterility was due to allelic interactionat each locus and that genetic effects of the two loci were additive.Wide-compatibilityvariety Dular has neutral alleles at loci qSS5 and S32(t) that can overcome thesterilities caused by these two genes.3, The BC₁F₂ was developed for further confirmation and fine mapping of S32(t). Thegene S32(t) was precisely mapped at the same region as that detected in the BC₁F₁ butits position was narrowed down to an interval of about 1.9 cM between RM236 andRM12475. By assaying the recombinant events in the BC₁F₃ with markers developedfrom the sequence information, S32(t) was further narrowed down to a 64kb region onthe same PAC clone. Sequence analysis of this fragment showed seven predicted openreading frames, four of which encoded known protein and three encoded putativeprotein. The results would be useful for map-based cloning of S32(t) andmarker-assisted transferring of the neutral allele in hybrid rice breeding.