| Through 50 years of development,hybrid rice has made an important contribution to China’s food security.China has successfully cultivated and popularized three-line hybrid rice since the 1970 s,which has greatly improved the per unit area yield of grain;then,in the1990 s,the two-line hybrid rice was successfully cultivated and popularized,and the yield of single season rice was from 10.5 t ha-1 to 15 t ha-1.Hybrid rice has been listed by the Food and Agriculture Organization of the United Nations(FAO)as the preferred technology to solve the problem of food shortage in the world.So far,it has been widely promoted in many countries.However,the study on the molecular mechanism of heterosis in hybrid rice lags far behind the utilization of heterosis,in addition the specific performance of main yield traits of hybrid rice in the paddy field are still not very clear.It is necessary to find the genes controlling heterosis and the main heterosis performance of hybrid rice through modern biotechnology and field cultivation experiments as well.In this study,the super hybrid rice LYP9 and its parents were taken as the main research objects,the key genes of yield heterosis in two-line hybrid rice were determined by analyzing the whole genome,transcriptome and phenomics.Through the same paddy field experiments,the main yield traits and other agronomice traits of super high-yielding hybrid rice varieties,genetic engineering hybrid rice varieties,and their restorer lines and inbred rice varieties were analyzed and compared.The main yield traits and other agronomice traits of early-season hybrid rice and inbred rice varieties,later-season hybrid rice hybrid rice and inbred rice varieties were also compared under customary(high)and combined(low)fertilization.These studies provide an important basis for the future development of hybrid rice.The main results were as follows:1)The heterosis gene RH8 in rice was identified by RIL(a recombinant inbred line)derived from LYP9 progenies and RILBC1(RIL population backcrossed with maternal parent).A high-density SNP marker linkage map of the LYP9-derived RIL population was constructed for detection of yield-related.A total of 27 QTL for all traits were mapped independently on rice chromosomes 1,2,4,6,8,10,11,and 12.Three of these QTL on chromosomes 2,4,and 8 were identified for SPP.Two QTL were detected for EPN on chromosomes 2 and 8.With regard to the QTL positions,two QTL clusters are highlighted: q SPP2/q EP2 and q HD8/q PH8/q SPP8/q YD8 are on chromosomes 2 and 8,respectively.The RILBC1 population were used to detect QTL for yield-component heterosis.Twenty-five loci for the respective phenotypes were detected on chromosomes1,2,3,6,7,8,10,11,and 12 in the 4-y results.Twelve of these loci overlapped with QTL detected in the RIL population,and 13 were detected independently in the RILBC1 population.One QTL cluster,qh HD8/qh PH8/qh SPP8/qh EP8,was found in the RILBC1 population,on chromosome 8.This cluster overlaps with the QTL cluster q HD8/q PH8/q SPP8 that was detected in the RIL population.One gene named RH8(rice heterosis 8)was identified with pleiotropic function in this cluster.RH8 is also the known rice gene Ghd8 that affects the number of grains per panicle,plant height and heading date.2)The differentially expressed genes between LYP9 and its Parents were found by transcription profiling in order to assisted determination of heterosis related genes of yield traits As GNP is the main yield advantage for hybrid rice.The transcriptomes of young inflorescence buds at the four key stages for the rice reproductive growth transition and 6 week-old leaf blades in the hybrid and its parents were determined to explore further the possible mechanisms and genes involved in the yield heterosis.15,843 differentially expressed genes(DEGs)out of 17,993 genes expressed in the panicle and 10,821 DEGs out of 15,145 genes expressed in the leaf between any two cultivars were identified.Several differentially expressed genes are located in the yield-related QTL regions.In the five QTL regions for SPP,673 genes were differentially expressed in LYP9 and its parents at the young inflorescence bud stages.Of these,17 overlapped with the QTL peak signals.3)The LYP9-like heterozygosity of RH8 is a notable feature shared by most two-line rice hybrids.The allelic combinations of RH8 in a set of 361 commercial hybrid rice populations comprising 125 two-line hybrids and 236 three-line hybrids were analyzed.RH8 heterozygosity,consisting of a strong allele and a nonfunctional allele,was found in 89 hybrids,of which 51 were two-line hybrids and 38 were three-line hybrids,accounting for 40.8% and 16.1% of the two-and three-line hybrids,respectively.Thus heterozygosity at the RH8 locus could be a significant feature,but not the only one,in commercial hybrid rice cultivars and particularly in the two-line hybrids.Ghd7,another flowering-time gene functioning parallel to Ghd8,has been found to contribute significantly to high yield heterosis.The heterozygosity consisting of a strong allele and a weak or nonfunctional allele also was found at the Ghd7 locus.4)It was confirmed that the grain number per panicle(GNP)was the major yield advantage trait of hybrid rice with longer growth period.The yield of different types of hybrid rice increased 10%-15% than that of male parent or inbred varieties.The yield advantage of hybrid rice was obvious,but not all yield trait components showed advantages.The average PPH(paternal parent heterosis)and HCK(standard heterosis,compared with inbred variety)for GNP of single season hybrid rice with longer growth period was 9.4%-15.27% and 22.87%-38.25%.The average PPH and HCK for EPN(effective panicle number per hill)of the single season hybrid rice with longer growth period was-0.4%-3.59% and-20.43%--12.65%.The average PPH and HCK for SSR(seed set rate)of the single season hybrid rice with longer growth period was4.65%-6.24% and-3.78%--1.44%.The average PPH and HCK for KGW(1000-grain weight)of the single season hybrid rice with longer growth period was-3.91%--2.79%and 7.81%-8.33%.The average PPH and HCK for HI(harvest index)of the single season hybrid rice with longer growth period was 11.52%-12.69% and-6.75--5.42%.The average PPH and HCK for yield of the single season hybrid rice with longer growth period was 10.62%-13.43% and 10.28%-14.48%.The average HCK for GNP of the later season hybrid rice was 4.29%-14.25%,and it’s yield traits are similar to the single season hybrid rice.The advantage of EPN for the early season hybrid rice was obvious.The average HCK for EPN of the early season hybrid rice was 7.25%-7.9%,but the advantage of its other yield traits was weaker.Our results show that the average EPN of the hybrid rice with Huazhan as paternal line is significantly higher than that of the hybrid rice with the 9311-type paternal line.It is noted that in general the performance of yield heterosis of hybrid rice is stronger under low fertilizer cultivation rather than customary cultivation.From the field investigation,the genetic engineering hybrid rice also shows strong positive advantages in GNP,KGW and yield,but not in it’s SSR and;on the whole,the yield advantage of genetically engineered hybrid rice is similar to that of two-line hybrid rice. |
