Dissection Of The Genetic Basis Of Heterosis For Yield-Related Agronomic Traits Between Intersubspecific Crosses Using Chromosome Segment Substitute Lines In Rice (Oryza Sativa L.)

With the development of social economy, greater demand was put on world food production due to the decreasing arable lands and increasing global population. The yield increase of rice, a main staple for a large fragment of the world population (amounting to half global population and 60%of population in our country), would contribute significantly to cope with the increasing global food crisis. Heterosis application was the principal way to improve rice grain yield.However, since the advancement of research on the genetic basis of heterosis was lagged behind the utilization in the field production in last century, which limited a larger scale of heterosis utilization in crop production. The research of heterosis in rice is not only meaningful to guide the rice hybrid breeding, but also provide a theoretical model for the research of heterosis in the other cereal crops because rice is a model plant in monocotyledonous genomic research. Therefore, it is very important to conduct the research of heterosis and dissect its genetic basis for the theoretical value and realistic meaning in hybrid breeding of rice.The extent of heterosis expressed in inter-subspecific crosses follows the general trend: indica/japonica> indicalindica> japonica/japonica. The extensive exploitation of inter-subspecific heterosis derive from indica/japonica is a very important strategy to enhance the yield level in rice production. The inter-subspecific cross between a japonica cv. Asominori and an indica cv. IR24 demonstrated huge heterosis in F1 hybrid. In our study, we conducted our experiments across four environments (2007,2008 in Nanjing; 2007, 2008 in Nanchang), mainly focus on the dissection and discussion of the genetic basis of heterosis about yield-related agronomic traits using two sets of chromosome segment substitution lines (CSSLs) derived from Asominori (a japonica cultivar) and IR24 (an indica cultivar) as reciprocal donor parent and recipient parent (background parent) and F1 derivatives from these two set of CSSLs population. QTL were detected by statistical software QTL IciMapping using two new different SSR map. The main conclusions are as follows:1. Genotypes of these two sets of 66 CSS lines (one is indica c.v. IR24 chromosome segment substitution lines with Japonica c.v. Asominori genetic background; the other is japonica c.v. Asominori chromosome segment substitution lines with indica c.v. IR24 genetic background) were rescreened by 137 and 132 good polymorphism SSR markers, replacing 82 original RFLP markers. The purpose of replacing with SSR markers was to simplify the procedure of experiments by using RFLP markers and to enhance the efficiency of QTL detection using new maps with higher density of SSR markers.2. Most of the agronomic traits showed transgressive segregation phenomenon in two CSSLs population; Heterosis of each agronomic trait exhibited a huge range of segregation, however different traits showed different level of heterosis; The studied traits also showed significant difference between years and genotypes, the interaction between genotypes and environments also exhibited a significance level.3. Not only the high value parent but also the low value parent has detected many alleles at QTL with positive or negative effects in the whole genome, some chromosome segments were tightly linked.4. We have detected 83 and 53 heterotic loci (HL) affecting yield component traits in Fi derivatives of AIS and IAS population, respectively. Of all the HL, only 13 HL could be detected repeatedly in two environments at least which reflected heterosis of yield component traits were very sensitive to environmental influence. Chances were the effect of pleiotropism, chromosome segments RM267 and RM82 on Chr.5 and Chr.7 could negatively affect grain weight plant-1 (GWP), grains per panicle (GPP) and seed-setting rate (SSR), simultaneously. RM82 could enhance the performance of sink capacity plant-1(SCP) and plant height (PH) as well. Chromosome segment RM5390 as an enhancing heterotic locus on Chr.2 could positively affect GWP, GPP and SSR traits simultaneously. RM252 and RM217 on Chr.4 and Chr.6 could decrease PH of F1 hybrids. Considering the effect of heterotic loci,39 with positive heterosis (amounting to 46.99%) and 44 (53.01%) with negative heterosis in AIS F1 derivative; 22 HL (41.51%) with positive and 31 (58.49%) with negative effect of heterosis in IAS F1 derivative. Possibly, we could create high-yield cultivar through pyramiding HL with positive effect and deleting HL negative effect by molecular tools such as SSR polymorphism markers. 5. We have detected 40 and 22 HL affecting panicle morphological traits in F1 derivatives of AIS and IAS population, respectively. Of all the HL were detected,27 with positive heterosis (67.50%) and 13 (32.50%) with negative heterosis in AIS F] derivative; 12 HL (54.55%) with positive and 10 (45.45%) with negative effect of heterosis in IAS F1 derivatives. Only 2 HL in AIS F1 derivative could be detected repeatedly in two more environments which reflected heterosis of panicle morphological traits were very sensitive to environment, too. Chromosome segments RM284 and RM331 on Chr.8 could increase the performance of panicle length (PL) and primary branches number (PBN), respectively. None HL could be detected in IAS F1 derivative. But we found that RM284 on Chr.8 could be detected in two sets of F1 derivatives, simultaneously.6. Through mapping additive QTL and dominance QTL affecting yield component traits, we could dissect the genetic basis of heterosis. Eighty-nine additive QTL and 79 dominance QTL affecting eight yield component traits (grains weight per plant, GWP; sink capacity per plant, SCP; spikelets per panicle, SPP; grains per panicle, GPP; seed-setting rate, SSR; thousand-grain weight, TGW; panicles per plant, PPP; plant height, PH) were detected across four environments in AIS and F1 derivative population; 117 additive QTL and 125 dominance QTL located on all 12 chromosomes affecting eight yield-related traits were detected across the four same environments in IAS and F1 derivative population. Through comparing dominance effects and additive effects of those 151 QTL totally affecting eight yield-related traits in AIS and its corresponding F1 population, we found 52 QTL with over-dominance effect (amounting to 34.44%),61 QTL with partial dominance effect (amounting to 40.40%),22 QTL with complete dominance (amounting to 14.57%), 16 QTL with additive effect (amounting to 10.60%); we also detected 181 QTL totally affecting five panicle traits in IAS and its corresponding F1 population, of all the QTL with additive and dominance effect,61 QTL were identified with over-dominance effect (amounting to 33.70%),69 QTL with partial dominance effect (amounting to 38.12%),32 QTL with complete dominance effect (amounting to 17.68%),19 QTL with additive effect (amounting to 10.50%). Results indicated that both dominance (mainly were partial dominance) and over-dominance effects of QTL may be as the major reason in underlying the genetic basis of heterosis of yield component traits in rice hybrids.7. We detected 52 QTL with additive effects and 55 QTL with dominance effects affecting five panicle morphological structural traits (panicle length, PL; density of spikelets per panicle, DSP; primary branches number, PBN; secondary branches number, SBN; density of secondary branches number, DSBN) across different environments in AIS and F1 derivative population. We found 72 additive QTL and 62 dominance QTL affecting five panicle morphological structural traits were detected across three same environments in IAS and F1 population. Comparing dominance effect and additive effect of those 82 QTL affecting five panicle morphological structural traits in AIS and its corresponding F1 population, we found 18 QTL with over-dominance effect (amounting to 21.95%),45 QTL with partial dominance effect (amounting to 54.88%),11 QTL with complete dominance (amounting to 13.41%),8 QTL with additive effect (9.76%); we also detected 108 QTL affecting five panicle traits in IAS and its corresponding F1 population, of all the QTL with additive and dominance effect,26 QTL were identified with over-dominance effect (amounting to 24.76%),47 QTL with partial dominance effect (amounting to 44.76%),18 QTL with complete dominance effect (amounting to 17.14%),17 QTL with additive effect (amounting to 16.19%). Results indicated that dominance (mainly were partial dominance) of QTL plays an important role in underlying the genetic basis of heterosis of panicle traits in rice hybrids.8. We have detected many QTL clusters affecting agronomic traits located on chromosome 1,2,3,4,6,7,8,9 and 12 maybe because of pleiotropic effects of QTL or close linkage of some markers. We also analyzed the genetic relationships between QTL clusters and yield-related agronomic traits and found pleiotropic QTL was a common phenomenon which exhibited the pleiotropic phenomenon might be one of the genetic bases of yield-related traits and heterosis.