| Rice (Oryza sativa L.) is a staple food for more than half of the world population. The increasing demand for rice in the world caused by population increase can only be met by enhancing yield per unit area under the environment that both arable land acrage and water resources are decreasing. Hybrid rice is a proven and successful technology for rice production over the past three decades. Seventeen million hectares of hybrid rice are grown in China, occupying about50%of total rice area planted annually. Hybrid indica rice accounts for70%of total indica rice area. The total area of japonica rice grown annually in China is8.4million hectares, and only3%is occupied by japonica hybrid rice. Japonica hybrids still have great potential to development. The major reason for this situation was that competitive heterosis of hybrid cultivar was not conspicuous in yield and quality, compared with conventional cultivar in japonica rice. The key of enhancing competitive heterosis of hybrid cultivar in japonica rice was to improve combining ability (CA) of yield related traits and quality traits of restorer lines. On the basis of previous studies, four aspects of research are carried out in this study. Firstly,72F1combinations were made from6cytoplasmic male sterile (CMS) lines and12restorer lines according to North Carolina Genetic Mating Design Ⅱ(NCⅡ). Eight yield related traits of the72F1combinations were investigated, and general combining ability (GCA) effects and variance of special combining ability (VSCA) of the8traits of the18parents was analyzed. SSR marker genotypes for elite CA of the eight traits, i. e., daily yield per plant (DYP), panicles per plant (PP), total spikelets per panicle (TSP), filled spikelets per panicle (FSP) and1000-grain weight (TGW), primary branch number per panicle(PBN), second branch number per panicle(SBN) and panicle length(PL), were screened by combining the SSR genotyping data with CA data of the18parents. Secondly,10quality traits, namely, brown rice rate, milled rice rate, head rice rate, unhulled grain length, grain width, percentage of chalky grain, chalkiness degree, gelatinization temperature, gel consistency, and amylose contents, of the72F1S made from the same6CMS lines and12restorer lines as above were measured. And the CA of the10quality traits of the18parents was analyzed. SSR marker genotypes for elite CA of the10quality traits were screened by the same method as mentioned above. Thirdly, SSR marker genotypes for elite CA of the grain yield related traits were re-identified by using phenotype values of91F1combinations made from7CMS lines and13restorer lines and genotype values of the20parents. The115SSR primers used for genotyping the20parents are the same as above. Among the7CMS lines and13restorer lines used in this part,6CMS lines and10restorer lines were identical to those used in previous report and in part1and2of this study. SSR marker genotypes for elite CA of the traits were compared with that in enlarged parent population. And consistency of SSR marker genotypes for elite CA of the grain yield related traits across years was analyzed using the same set of parents and the same set of SSR primers. Forthly, improvement of CA for productive panicles per plant and gel consistency in a restorer line Ninghui8and for daily average grain yield in another restorer line157TR-68were carried out by using hybridization, backcrossing and assisted selection of SSR markers identified in this study. The main results obtained were the follows:1. Thirty-one SSR marker genotypes were significantly associated with CA of the eight traits, using the dataset of72F1s made from the six CMS lines and twelve restorer lines. Out of the thirty-one, fifteen were associated with one trait, three for two traits, five for three traits, four for four traits, two for five traits, and two for six traits. Among the four marker genotypes associated with four traits simultaneously, RM23-150/160showed positive effects of CA in all the four traits, increasing11.6%of total spikelets per panicle,11.2%of daily yield per plant,10.1%of panicle length and15.0%of second branch number per panicle in F1, respectively. Among the five marker genotypes associated with three traits simultaneously, three showed positive effects of CA for all the three traits.2. A total of30SSR marker genotypes were significantly associated with CA of the10quality traits, namely, brown rice rate, milled rice rate, head rice rate, grain length, grain width, percentage of chalky grain, chalkiness degree, gelatinization temperature, gel consistency, and amylose contents, using the dataset of72F1s made from the same6CMS lines and12restorer lines as above. Twenty-five of them were marker genotypes for unfavorable CA of quality traits. The remaining five marker genotypes were associated with elite CA for quality traits of parents. RM263-175/180and RM444-230/240were marker genotypes of elite CA for head rice rate, which increased3.2%and2.5%of head rice rate in F1, respectively. RM3-120/150shortened grain length value in F1by2.4%. RM444-180/240increased grain width trait value of F1by2.1%. RM428-273/294decreased amylose content trait value of hybrid rice harvested from F1plants by7.0%. Among the SSR marker genotypes associated with CA of the10quality traits, eight marker genotypes also affected CA of yield components simultaneously. RM3-120/150increased simultaneously total spikelets per plant and filled spikelets per plant in F1by15.9%and10.9%, respectively. RM1211-150/160decreased trait values of brown rice rate and milled rice rate in F1by0.9%and1.1%, respectively, and simultaneously increased trait values of total spikelets per plant and filled spikelets per plant in F1by21.8%and20.4%. RM23-150/160increased trait values of percentage of chalky grain and chalkiness degree in F1by44.1%and45.7%, and simultaneously increased daily yield per plant and total spikelets per plant in F1by11.2%and11.6%, respectively.3. Sixty-six SSR marker genotypes were significantly associated with CA of9yield related traits, namely, DYP, PP, TSP, FSP, TGW, days to heanding (DTH), plant height (PH), PL and grains per cm panicle (GPCP), using the dataset of91F1s derived from7BT-type japonica male sterile lines and13restorer lines in rice made with NCII design. Among the66SSR marker genotypes detected, Twenty-two were significantly associated for only one trait, nineteen for two traits, eight for three traits, ten for four traits, three for five traits and four for six traits. Comparing the marker genotypes identified from the identical16parents and115SSR markers used in2009and in2010,26marker genotypes were found associated significantly with the same trait in both years. Among them, four for PP, eight for TSP, eleven for PL, three for FSP.4. By hybridizing, bachcrossing, selfing and testcrossing.12new restorers with180bp homozygous at marker locus RM208in background of Ninghui8were obtained. Among them, combining ability for productive panicle per plant in8010-4-24,8010-6-5and8010-6-23increased by45%averagely, compared with that of check restorer Ninghui8, when crossed with CMS line Wu3A. And CA for productive panicle per plant in8010-4-10,8010-4-14and8012-2-9enhanced by65.5%,56.9%and86.2%, respectively, compared with that of Ninghui8, when crossed with CMS line863A. Six new restorers with96bp homozygous at marker locus RM5556in background of Ninghui8were obtained. CA for gel consistency of milled rice in8009-14-3,8009-14-18,8009-14-29,8013-22-10and8013-22-24was significantly lower than that of check restorer Ninghui8, when crossed with CMS line9522A. Eight new restorers with170bp homozygous at marker locus RM152in background of Ninghui157were bred. CA for daily grain yield per plant in8014-26-1,8014-26-12,8014-26-21,8014-26-24,8015-7-4and8015-7-17was significantly higher than that of check restorer Ninghui157, when crossed with CMS line Liuqianxin A. Daily grain yield per plant of the6F1increased by86.8%,43.1%,78.5%,55.6%,91.0%and82.6%, compared with check Liuyou157.Summaring the screening results, we found many SSR loci associated with CA of yield related traits in parents, and about33%of the loci were associated with two or more than two traits. Those loci associated with CA of multi-traits consistent with breeding objectives might facilitate CA improvement for multi-traits simultaneously. Marker genotypes that were repeatly detected across years and were associated significantly with CA of the same trait showed the same effect to F1trait value in direction, and the percentage of increment was almost the same in different years. Expect results may be obtained by using the stable marker genotypes across years for improving CA of restorer lines in japonica rice. Among the SSR marker genotype loci associated with CA of quality traits,80%of the heterozygous loci were unfavorable to quality of milled rice obtained from the F1plants. This phenomenon may be related to the genotype segregation of grains harvested from from F1plants. Thus two parents with identical alleles at loci affecting quality traits are desirable. The efficiency of improving CA of yield related traits was better than that of quality traits by allele substitution according to information of elite CA marker genetyps. Wu3A/8010-4-24and863A/8012-2-9had more productive panicles per plant and higher daily average grain yield than their corresponding check cultivars, and there was no significant difference between the combinations and their checks in plant height and growth duration. These two combinations, therefore, are hopeful to be used in production. |
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