| Increasing yield potential has been one of the most important breeding objectives worldwide, especially for China which has the largest population but limited land. Knowledge on genetic gain of yield potential and its associated traits is essential for understanding yield-limiting factors and developing strategies for future variety improvement. The objectives of this study were (1) to understand genetic gain for grain yield, plant height, and physiological traits and to identify the limiting factors for improving wheat yield potential in Shandong province, (2) to determine associations between kernel traits and molecular markers and to identify QTLs affecting kernel traits in a RIL population derived from the cross PH 82-2/Neixiang 188, and (3) to assess the associations between traits of interest and genetic markers by the linkage disequilibrium and to find associations with stripe rust resistance and yield traits in elite parent Zhou 8425B and its derivatives.1. Fifteen genotypes including 13 milestone varieties released from 1969 to 2006 and two advanced lines were examined over three years in the field experiments at Tai'an during the 2006-2009 cropping seasons. The mean genetic gain in grain yield was 62 kg ha-1 yr-1 or an annual rate of 0.85%, largely due to increased kernels m-2, reduced plant height, increased above-ground biomass, and increased harvest index (HI). Significant genetic changes were also observed for chlorophyll content (Chl) at anthesis (0.26% yr-1), leaf area index (LAI) at heading (-0.53% yr-1) and at anthesis (-0.45% yr-1), photosynthesis rate (Pn) during grainfilling (0.47% yr-1), and stem water-soluble carbohydrate (WSC) content at anthesis (0.76% yr-1). All except stem WSC showed significant associations with grain yield, with r = 0.62, -0.68, -0.74, 0.69 and -0.55, respectively. In comparison with the genotype Rht-D1b, Rht-D1b+Rht8c showed increased kernels spike-1, kernel weight spike-1, HI, canopy temperature depression (CTD) at heading and LAI at heading and anthesis. Increasing the number of kernels per spike by reducing the number of sterile spikelets per spike, improving the utilization of photosynthetic production, then combining greater weight of grains per spike with higher number of spikes per squire meter, is one of the best approaches in breeding for further yield increase.2. To map the quantitative trait loci (QTLs) for kernel morphology traits, 240 recombinant inbred lines (RILs) derived from a cross between non 1BL.1RS translocation variety PH 82-2 and a 1BL.1RS translocation variety Neixiang 188 were grown at three locations (i.e. Tai'an of Shandong Province, and Jiaozuo and Anyang of Henan Province) for two years. Inclusive composite interval mapping identified 71 main-effect QTLs on 16 chromosomes for seven kernel morphology traits, viz. kernel length (KLEN), kernel width (KWID), kernel perimeter (KP), kernel area (KA), kernel width/length ratio (KW/L), shape factor (SF) and factor form-density (FFD), measured by digital imaging (DI). These QTLs were mapped on chromosomes 1A, 1B, 1D, 2A, 2D, 3A, 3B, 4A, 4B, 5A, 5B, 5D, 6B, 6D, 7A and 7B, respectively, explaining 2.6 to 28.2% of the phenotypic variation. Eight QTL clusters conferring the largest effects on thousand kernel weight (TKW) and kernel morphology traits were detected on chromosomes 1BL.1RS (2), 2A, 4A, 4B, 6B, 6D and 7A. Fourteen epistatic QTLs were identified for all kernel morphology traits except KW/L, involving in 24 main-effect QTLs distributed on 13 chromosomes, explaining 2.5 to 8.3% of the phenotypic variance. Five loci, viz. Sec-1 on 1BL.1RS for KWID, KP and KA, Glu-B1 on 1BL for KLEN and KP, Xcfe53 on 2A for KLEN, KP and KW/L, Xwmc251 on 4B for KLEN, SF and KW/L, and Xbarc174 on 7A for KWID, KP, SF and KW/L, were detected consistently across locations and seasons, and their linked DNA markers may be used for marker-assisted selection in breeding for improving wheat kernel traits and grain yield.3. The elite parent Zhou 8425B and its 50 derivative varieties and lines were used to compare their genetic structure and components among different generations derived from Zhou 8425B, and association mapping was employed to identify stripe rust resistance loci along the whole-genome with 921 Diversity Array Technology (DArT) and 83 SSR markers. The results showed that the averaged genetic similarity index was 74.6% among Zhou 8425B and its derivatives, and the clustering results for these genotypes were basically identical with the pedigree analysis. On the whole genome, Zhou 8425B had the contribution ratio of 67.7%, 63.6% and 58.8% to its first, second and third generation, respectively. The genetic contribution ratios on A, B and D genomes were 68.7%, 62.0% and 59.4%, respectively. On single chromosome level, Zhou 8425B alleles could be detected in the derivatives from 44.9% (4A) to 70.9% (1D). The different contribution ratios derived from the elite parent on genome and chromosome levels in different generations were probably caused by various important alleles associated with agronomic traits and resistance genes.4. The elite parent Zhou 8425B and its 50 derivative varieties and lines were seeded in a completely randomized block design with three replications in Anyang and Zhoukou during 2007-2008 and 2008-2009 seasons. Yield traits were measured at physiological maturity and the maximum disease severities (MDS) of stripe rust were recorded. A total of 1214 alleles were used to find associations with yield traits and stripe rust resistance. A total of 73 alleles located on chromosomes 1A, 1B (3), 2A (2), 3A, 3D, 4A, 4D, 6B and 7D were significantly associated with grain yield, kernels per square, thousand kernel weight, spikes per square, kernels per spike, spike weight and plant height, explaining 14.4-32.8% of the phenotypic variation. Twelve QTL clusters conferring the yield traits were detected on chromosomes 1A, 1B (3), 2A (2), 3A, 3D, 4A, 4D, 6B and 7D. Four loci were detected to be associated with stripe rust resistance by association mapping (P < 0.01), in which two loci QYr.caas-2BL and QYr.caas-7BL were found in the common genomic regions of Yr7 and YrZH84, respectively. A new QTL QYr.caas-1BL was found in the same genomic region as leaf rust resistance gene LrZH84, which might be attributed to be pleiotropic effect or tight linkage each other. Another new locus QYr.caas-3AL was located in the terminal region of the long arm of chromosome 3A, explaining 22.9% of the phenotypic variance. In addition, most of associated markers were found in genomic regions where previous reports had found genes or QTL influencing the same traits, providing an independent validation of this approach. The significantly associated alleles with yield traits and stripe ruse resistance would be important for new varieties development in the Yellow and Huai Valleys Winter Wheat Zone. |
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