Genetics Of Seeds Per Silique And Mapping Of Major QTL In Brassica Napus L.

Seeds per silique (SS) is one of the three important components of yield in oilseed rape (Brassica napus L.) and has always received much attention. By comparing SS in the representive cultivars registered officially in China, it could be inferred that SS was about 20. In the breeding practice, one stable high-SS and high combining ability line'Y106'(or 206A), was found by Rapeseed Laboratory of Huazhong Agricultural University. This elite line had been applied to produce many hybrid cultivars, i.e.'Shengguang 86','Huawanyou No. 5','Huayouza No.15'and so on, registered officially in nation or province trials. And the SS of these hybrids was above 23 in the farmers'fields. Although'Y106'had remarkable high-SS effect in the breeding programs, the genetic basis of SS remains elusive. Hence, genetic dissection of SS will facilitate formulating breeding strategies for seed yield improvement.The Brassica napus lines,'Y106'exhibiting high-SS and'HZ396','HZ165'and'HZ168'with low to moderate SS, had different genetic backgrounds and were used as parents. We analyzed the genetic studies of SS and mapped the major QTL cqSS.N19 in a B3F₂ population. The main results were as follows.1. The genetic control of SS was primarily studied in the cross of'HZ396'×'Y106'. The frequency distribution of the F₂ generation deviated from a normal distribution and appeared to have a multi-modal pattern, indicating the influence of major genes mixed with polygenes.2. To test this hypothesis and uncover the genetic basis of SS in diverse genotypes, three low to moderate SS materials,'HZ396','HZ165'and'HZ168', were selected. An experiment with the orthogonal and reciprocal F₁ generations derived from three crosses,'HZ396'×'Y106','HZ165'×'Y106'and'HZ168'×'Y106', along with their parents was conducted for investigating the heterosis and cytoplasmic effects of this trait. Results revealed that SS more or less unaffected by environments, that SS was controlled by nuclear genes instead of cytoplasmic genes, that there were all levels of dominance from partial to full dominance in the F₁ as the major contributor to heterosis and the higher-SS genotype was almost completely dominant over the lower-SS genotype.3. The major genes and polygenes mixed genetic model was used to analyze SS in rapeseed. The joint segregation analysis revealed that SS was best described by the E-0 genetic model, a case of two additive-dominace-epistasis major genes as well as additive-dominace-epistasis polygenes. The additive effects of the two major genes ranged from 1.49 to 3.71 in the three crosses. And the additive effects of two major genes in the cross of'HZ396'×'Y106'were obviously larger than those in the other two crosses, which demonstrated that the two major genes for SS in the cross of'HZ396'×'Y106'could easily be detected and passed on to offspring. Significant progress could be made in mapping the two major genes contributing to SS by constructing mapping populations using'HZ396'and'Y106'as parental lines. Heritability of the major genes varied from 39% to 84% in diverse generations (B1, B2 and F₂), which was much larger than that of the polygenes. It meant that SS was mainly controlled by major genes.4. Allelic tests of the three low to moderate SS materials,'HZ396','HZ165'and'HZ168'were performed by the frequency distribution of the F₂ generations. Results revealed that the three lines were probably non-allelic, which meant that the major genes varied according to the specific crosses performed.5. To dissect the genetic basis of SS, we used one F1 plant of the cross of'HZ396'×'Y106'to develop double haploid (DH) population. The DH population, consisted of 140 lines, was used for map construction and QTL analysis. A linkage map comprising 151 Simple Sequence Repeat (SSR) and 198 Amplified Fragment Length Polymorphism (AFLP) markers covering 1833.9 cM with an average interval of 5.1 cM between adjacent markers was constructed. The order of most markers is consistent with the published linkage maps of Piquemal et al. (2005) and Cheng et al. (2009).6. In field experiments across three seasons and two locations in China 140 doubled haploid lines and their corresponding parents were evaluated for silique-traits. Quantitative Trait Loci (QTL) meta-analysis revealed that 6, 3 and 4 consensus QTL for silique length (SL), SS and seed weight (SW) respectively. Of them, 9 QTL showed main effects. And 6 unique QTL were pleiotropic and mapped on linkage groups N7, N8, N13 and N19, which reflected significant correlation of all pairs of the silique-traits by the genomic location and effects of QTL detected. For the unique QTL in the linkage group N19, the additive effects of cqSS.N19, which explained 57.77% of the phenotypic variance of SS, and cqSL.N19 were positive while the additive effect of cqSW.N19 showed a negative effect at the same locus in the linkage group N19, which is a hint for the reason of positive correlation coefficient between SS and SL while negative correlation coefficient between SS and SW.7. Since the major QTL cqSS.N19 (for simplicity, designated as qSS.C9) was stable across various environments, we selected this QTL as a target QTL to map. Given that AFLP has limitations in genotype analysis, we attempted to convert the two AFLP markers EA08MC13-150 and SA09MC04-220 into SCAR markers. And only SA09MC04-220 successfully converted into codominant SCAR marker SCC9-005. To enrich the markers located in the target QTL further, we utilized the SSR markers in the published linkage maps of Chen et al. (2007) and Qiu et al. (2006) and developed SSR markers from the end sequence of BAC in the linkage group A10 in Brassic rapa. Results revealed that 3 SSR markers (sS2066, sNRG42 and SRC9-022) were enriched in the linkage group N19.8. For the consensus QTL, cqSS.N19, we constructed one near isogenic-line (BC3F2) in the HZ396 background by consecutive backcrossing, according to the primary QTL identified in DH lines. QTL analysis based on the BC3F2 population showed that this locus was located between the dominant SSR marker SRC9-022 and co-dominant SCAR marker SCC9-005. The QTL peak was near SRC9-022 marker at a distance of 0.60 cM. Furthermore this locus explained 85.8% of phenotypic variance with additive and dominant effects of 6.1 and 5.7 SS, respectively. The finding suggested that the locus was major for SS of'Y106', which had remarkable high-SS effect in the breeding programs.Three aspects of research work in the future are as follows: 1. Fine mapping and cloning of major QTL cqSS.N19; 2. Cloning of cqSW.N19 using recombinants based on cloning of cqSS.N19; 3. Expanding and deepening the correlation of all pairs of the silique-traits at the QTL level, especially packing of seed within the silique; 4. Genetic analysis of pyramiding QTL of SS.