Fing Mapping Of A Major QTL For 1000-Seed Weight On Linkage Group 7(A7) In Brassica Napus

Based on the initial QTL mapping for 1000-seed weight using SG-DH population (282 lines) over 9 environments, one major QTL located on linkage group 7 (A7) was selected for further fine mapping. This QTL was stably detected in 7 of 9 environments with LOD value from 3.35 to 16.75 and explained the phenotypic variation in population from 4.90% to 17.90%. This QTL showed an average additive effect of 0.148g and was flanked by markers EST12a and ZAAS864 (40.3cM).In present study, the major QTL for seed weight on A7 was validated and further fined mapped through the following procedures.1) QTL near isogonics lines including BC3F1/BC3F2 and BC4F1/BC4F2 generations covering target genomic region were developed by marker-assistant backcross; 2) 1138 and 594 single plants from BC3F1 and BC4F1 populations were individually isolated and subsequently analyzed by the markers within QTL region to get marker genotypes. For increasing the marker density, the positional specific markers were developed by using the sequence information from A7 of Brassica rape; 3) the corresponding selffing progenies of BC3F2 and BC4F2 families were employed for field trials with replications to obtain the trait phenotypes. The main results were as follows:1. Increasing markers in the target QTL intervalBased on the initial QTL mapping for 1000-seed weight on A7, hundreds of positional specific markers were developed in the QTL target interval by using the sequences information of A7 in Brassica rape. Of them,44 markers could be mapped within the QTL region and therefore, greatly increased the marker density from 19 to 63 markers with the average distance between two markers from 2.12cM to 0.60cM.2. The validation of the target QTL on A7The QTL for 1000-seed weight on A7 was validated by using the marker genotypes from BC3F1 single plants (n=1138) and trait phenotypes from their corresponding selfing progenies BC3F2. The results showed clearly linkages between three marker genotypes and trait phenotypes in each of marker loci within QTL region. Further, a significant difference of 0.190 g (P=0.001) for 1000-seed weight between homozygous BC3F2 sister sub-NILs carrying "Sollux" fragment (n=101, 3.169g) and NILs containing "Gaoyou" segment (n=85,2.979g) in the target region were observed. By comparison analysis among near isogonics lines in this step, the QTL region was narrowed from 40.cM to 9.0cM3. The further fine mapping of A7-QTL for seed weightThe further fine mapping for A7-QTL of 1000-seed weight was carried out by using the BC4F1/BC4F2 plants/families. Marker genotypes were analyzed with BC4F1 single plants (n=594), and trait phenotypes were obtained from their corresponding selfing progenies BC4F2. The same strategy as used in the populations of BC3F1/BC3F2 was adopted to conduct the linkage and comparative analysis. The results further narrowed the QTL from 9.0cM to 6.9cM region and fixed it between markers sR0282R and ZAAS864. Notably, the larger difference with 0.231g (P=0.009) for 1000-seed weight between two homologous genotypes of NILs carrying "Sollux" fragment (n=44,3.234g) and NILs containing "Gaoyou" segment (n=51,3.003g) in the 6.9cM region than identified in populations of BC3F1/BC3F2 were observed. This shows a trend when the genetic background are more identical between two QTL-NILs, the additive effect of the QTL might increase and therefore to further indicate the potential for map-based cloning and for breeding purpose of this QTL.