Mapping Quantitative Trait Loci For Boron Efficiency In Brassica Napus

Nutrient efficient crops which produce higher yields per unit of nutrient applied or absorbed is a latest important breeding target. Rapeseed is the second important oil crop after soybean. Brassica napus was the most important member of rapeseed which occupies about80%of rapeseed area. Boron (B) is one of essential micro-nutrient for high plant growth. B deficiency becomes a various limiting factor of increasing crop yield in the world. B. napus which is in great demand of B shows very sensitive to B deficiency. B deficiency could lead to flowering without seed set and yield decrease even no seed setting. Application of borax fertilizers can alleviate the problem in some degree, but can also lead to environmental problem, and B ore resources are limited. It was reported that considerable genotypic variation exists among different cultivars in response to B deficiency in B. napus. So, developing B efficiency cultivars by genetics and biotechnology is a essential approach for improving B. napus in response to B deficiency. In this study, a genetic linkage map was constructed based on the B. napus BQ DH population which was developed by other members in our group. Three-year field trials with low and normal B treatments were conducted using the BQ DH population for investigating seed yield and yield-related traits. Quantitative trait locus (QTL) analysis for seed yield and yield-related traits under low B and normal B conditions using the BQ DH population was conducted. Main results are as follows:1. Construction of B efficiency BQ DH genetic linkage map in B. napusA total of1413SSR primers named "BnGMS" and "BoGMS" from other study reports,403SSR primers developed by our group according to sequenced BAC of A2, A3and A9and50SSR primers developed according to sequence of A2chromosome in Brassica rapa in this study were used for genotyping in the BQ DH population. A genetic linkage map named as BQ DH map was constructed based on the BQ DH population by software Joinmam4.0. BQ DH genetic map comprise a total of486molecular markers, including468SSR markers,7SRAP markers,9GBM markers and2other markers, which covers a total length of1873.9cM with an average interval of3.86cM between adjacent markers. 2. Phenotype variations of the BQ DH population and their parents under different B conditionsTrait associated with B deficiency symptom at the full-bloom stage and filling stage under low B condition, seed yield and yield-related traits under both B conditions were investigated by three-year field trials using the BQ DH population and its parents. For the trait associated with B deficiency symptom, B-efficient cultivar QY10showed almost normal flowering and seed setting and almost no B deficiency symptom, B-inefficient cultivar Bakow was seriously influenced by B deficiency in flowering and seed setting and leaves. It indicated that QY10has a stronger adaptability to B deficiency than Bakow. Abundant variation among the BQ DH population ranged from grade I to grade V was observed. At the mature stage, significant differences were observed between the two parents by seed yield, plant height, branch number, pod number per plant, seed number per pod and seed weight under low B condition. And significant differences were observed between the two parents by seed yield, pod number per plant and seed weight under normal B condition. All the traits investigated under both B conditions showed continuous phenotypic variation and significant transgressive segregation in both directions, indicating these traits are all quantitative traits and controlled by multiple genes. Correlations among all the traits under single B condition were different. ANOVA results suggested that genotype, B level, environment and the interactions among these variable had significant effect on the six measured traits.3. QTL detection for seed yield and yield-related traits and index of boron deficiency (GBD) in the BQ DH populationSeed yield, plant height, branch number, pod number per plant, seed number per pod and seed weight under both B conditions and B efficiency coefficient (BEC) were use for QTL mapping in the BQ DH population. A total of70putative QTLs distributing on15linkage groups (except for A5, A8, C1and C2) singly explaining4.15-23.16%of phenotypic variation were detected by software WinQTLCartographer2.5, including30QTLs under low B condition,35QTLs under normal B condition and5QTLs for BEC. Among these QTLs,7and16QTLs were identified across three and two trials, respectively. By BioMercator2.1,29QTLs for seed yield and yield-related traits from other populations were projected onto the BQ DH population, co-locating with16QTLs for the same trait. Three QTLs for GBD were detected distributing on A7and C8linkage groups.4. Detection of epistatic interactions for seed yield and yield-related traits in the BQ DH populationSix measured traits under both B conditions and BEC were used to detect epistatic interactions in the BQ DH population by software QTLNetwork2.0. A total of32epistatic interactions distributing on the whole genome (except for A8) were detected, singly explaining0.53-14.26%of phenotypic variation. Most of them were the interactions between two loci without main-effect. It indicated that many epistatic interactions between two without main-effect loci are involved in regulating seed yield and yield-related traits under low and normal B conditions.5. Comparative mapping between the BQ DH genetic linkage map and Arabidopsis genome and in silico mappingThe comparative mapping between B. napus and Arabidopsis was carried employing SSR and GBM markers with known sequence information as anchored markers based on identified24conserved chromosomal blocks on Arabidopsis genome. A total of20synteny blocks and123insertions fragment islands were identified. By in silico mapping,153genes associated with seed yield, yield-related traits and B metabolism were mapped to the confidence intervals of70putative QTLs in the BQ DH population. The information and candidate genes were benefit for further study.6. Identification of B efficiency major locus BnBEl and prediction of candidate genes of BnBElComparative analyzing the QTLs for B efficiency on A2linkage group in the BQ F2, TN DH and BQ DH populations, the QTLs for B efficiency of the three populations co-localize in the same genomic region, just was the reported BE1. And the major locus for B efficiency at the mature stage was designed as BnBEl. Six candidate genes of BnBE1were predicted using known B. rapa genome information, including small molecule transporter genes, and B. rapa homologous genes of AtNIP5;1and AtNIP7;1in Arabidopsis.