Fine Mapping Of Thousand Seed Weight QTL TSWA7b In Brassica Napus L.

Seed size or weight is the key component of yield traits in Brassica napus L. and it directly affacts the yield of rapeseed. So far none of seed weight genes has been cloned in rapeseed and the regulatory mechanism of its seed weight is unknown. In previous study, we detected 9 QTLs of thousand seed weight trait in B. napus, including two major QTLs on A7 chromosome, TSWA7 a and TSWA7 b, which could explain 27.6%-37.9% of the total trait variations. We constructed the near isogenic lines（NIL） of TSWA7 b on the lower part of A7 chromosome. Based on the results, we further increased the identity of the genetic background in NIL. We also further fine mapped the TSWA7 b with the BC4F2 and BC3F3 NIL populations. These results could provide molecular markers closely linked to seed weight trait for rapeseed breeding, and also provide condition for revealing the molecular regulatory mechanism of seed weight. The main results of the research are as follows:1. In preliminary constructed BC3F1 NIL materials, we selected the plants whose target segments of the QTL TSWA7 b were heterozygous genotype and background response rates were the highest to get BC4F1 and BC5F1 materials with DH22 as female parent backcross. At the same time, we made use of 35 molecular markers for foreground genotype analysis identificating and screening the BC4F1 generation（n = 56） and BC5F1 generation（n = 61） NIL materials at TSWA7 b loci.2. The BC4F2 population including 229 individuals were obtained by self-fertilizing BC4F1 materials of TSWA7 b loci and the BC3F3 population including 301 individuals were obtained by self-fertilizing BC3F1 materials of TSWA7 b loci continuously. The local genetic linkage maps of the two populations were constructed with the newly developed In Del markers and existing SSR markers of QTL TSWA7 b loci genotyping analysis. The genetic linkage map of the BC4F2 population was 28.5 c M in length with 0.8 c M of the average distance between molecularmarkers including 35 molecular markers; The genetic linkage map of the BC3F3 population was 33.4 c M in length with 1.1 c M of the average distance between molecular markers including 31 molecular markers. The relative positions of the same genetic molecular markers were consistent totally on the two genetic maps.3. Silique length, seed number per siliqueand and thousand seed weight were measured in the BC4F2 and BC3F3 populations. In the two populations, three traits showed continuous variable separations, and a significant positive correlation was found between silique length and seed number per silique while a significant negative correlation was found between thousand seed weight and seed silique length or seed number per silique. The QTL scanning found that only the QTLs which controlled thousand seed weight trait were detected and their mapping intervals were also close basicly in the two populations: The QTL of thousand seed weight was located between the molecular markers SSR579 and BA7₂8 in BC4F2 population and it was located between the molecular markers Bn A7₂0 and Bn A7₂5 in BC3F3 population. The two QTLs detecting results showed additive effects of 0.100 g and 0.084 g respectively, contribution rates of 4.98% and 8.62% respectively, and the allele from DH78 played a role in increasing thousand seed weight. Comparing A, H and B genotype of target segment corresponding three traits with each other multiply in the two populations, we discovered that TSWA7 b locus controlled the increase of thousand seed weight when it would not affect seed silique length or seed number per silique.