Construction Of Chromosome Segments Substitution Lines And The Genetic Basis On Heterosis For Yield-Related Traits

Up to now, rice is the second largest food crop in china. Rice yield improvement guarantees the food security,which feeded the more than half world’s population. So the development of high-yield varieties is one of the most important objectives in rice breeding. Heterosis, or hybrid vigor, refers to the superior performance of the hybrids relative to the parents. Utilization of heterosis has contributed tremendously to the increased productivity in many crops for decades. Although there have been a range of studies on various aspects of heterosis, the key to understanding the biological mechanisms of heterotic performance in crop hybrids is the genetic basis, much of which is still uncharacterized. Shanyou63is an elite hybrid, in this study, we used original parent Zhenshan97and Minghui63of Shanyou63as the parents, a series of CSSLs were developed following the consecutive backcrossing strategy to study the genetic effects of Minghui63segments on yield and yield-related traits in the genetic background of Zhenshan97. In order to characterize the effects of Minghui63segments on heterosis, the population of CSSLs/Zhenshan97was used. In addition, we we generated three sets of reciprocal F1hybrids of indica and japonica subspecies to evaluate the relationship between yield heterosis and the circadian clock. The major results are as follows:1. A total of202CSSLs were constructed, each carrying one to three substitution segments. The average percent recovery of recurrent parent genome is95.2%. The total length of all substitution segments was2367.5cM, which was1.5times of the rice genome. The overlapped substitution segments covered1420.3cM (93.2%) of the entire rice genome. The estimated length of single introgression segments in CSSLs ranged from1.2to30.7cM with an average of11.2cM.2. Phenotypic evaluation of CSSLs was carried out in two environments. The traitscharacterized are heading date (HD), plant height (PH), panicles number per plant (PN), panicle length (PL), grains per panicle (GPP), spikelets per panicle (SPP), kilo-grain weight (KGW) and yield per plant (YD). Transgressive segregations observed in all these traits implied that the introgressed Minghui63segments had an effect on agronomic traits.3. The genetic effects of Minghui63segments composed of additive and dominant. Genetic effects differentiated among different introgression segments, epistasis interaction was significant contribution to phenotypic variation based on epistasis analysis. These results suggested that additive dominant and epistasis played an important role in trait performance.4. CSSLs/Zhenshan97was developed by selecting CSSLs as female and Zhenshan97as the male parents, to study the effects of Minghui63segments on heterosis. Eight traits were evaluated in two environments. Taken all these traits into consideration,17.8percent of hybrids significantly deviated from SY63, and all heterosis loci showed partial dominance. In2010, there were5.7,14.3and5.0percent of chromosome segment substitution lines for grain yield, Spikelet per panicle and grain per panicle, respectively.5. Twelve QTLs were identified for heading date. Single QTL individually explained5.2%to83.5%of heading date variation. Five QTLs were not detected in both F2and RIL populations from the same cross Zhenshan97/Minghui63. Parental comparative sequencing showed that the three major QTLs of QHD6, QHD7.2and QHD7.3are allelic with the cloned genes of Hdl, Ghd7and Ghd7.1, respectively. QHD7.1and QHD2are novel QTLs. The QTLs of QHD2, QHD4and QHD8with additive effects of about4.5d would be worthy to conduct QTL cloning. In addition, di-genic interaction between QHD7.2and QHD7.3significantly regulated heading date under long-day conditions. The diverse performance in agronomy and biology between the parents Zhenshan97and Minghui63indicated that the set of CSSLs would be idea materials for QTL mapping and cloning for other traits.6. In this study, Fifteen CSSLs had varied plant heights within lines. A total of15partial dominance QTLs for plant height were detected in these15CSSL-F2populations. All hybrids between the15CSSLs and the recurrent parent, Zhenshan97, were shorter than the corresponding CSSLs, but taller than Zhenshan97. These indicated that these15QTLs were also heterosis loci (HLs) contributed to heterosis acted in dominance. Each HL contributed from-7.4to14.4%of midparent heterosis. Additive by additive (AA) and additive by dominance (AD) interactions were detected in the Tetra-F2population segregating at the four major QTLs with the largest effects on plant height. Substantial negative AA effects were detected between two major QTLs qPH7.2and qPH7.3, which increased heterosis in the study.7. To understand the genetic basis of heterosis, we developed a set of202chromosome segment substitution lines (CSSLs) of an elite hybrid, Shanyou63.Thirteen and nine partial dominance QTLs for spikelets per panicle and yield were detected. Without exceptional, All hybrids between the9CSSLs and the recurrent parent, Zhenshan97, had less trait values than the corresponding CSSLs, but more than Zhenshan97. These indicated that these9QTLs acted in dominance and contributed to heterosis. Conversely, we generated two4-way F1s and one8-way F1with the genetic background of Zhenshan97. The heterosis of8-way F1reached up to75.4%that of Shanyou63for grain yield. Additive by additive (AA), additive by dominance (AD) and dominance by dominance (DD) interactions were detected in the both Tetra-F2populations each segregating at four major QTLs with the large effects on spikelets per panicle. Substantial negative AA effects were detected between two major QTLs HYD7.1and HYD7.2, which increased heterosis in the study.8. In this study, we generated three sets of reciprocal F1hybrids of indica and japonica subspecies to evaluate the relationship between yield heterosis and the circadian clock. There were no differences in trait performance or heterosis between the reciprocal hybrids, indicating no maternal effects on heterosis. The indica-indica and indica-japonica reciprocal F1hybrids exhibited pronounced heterosis for chlorophyll and starch content in leaves and for grain yield/biomass. In contrast the japonica-japonica F1hybrids showed low heterosis. In the hybrids that showed very strong better-parent heterosis (BPH) for grain yield, the investigated three circadian clock genes were expressed in an above-high-parent pattern during the vegetative period35days after sowing.