Rapid improvement of seed weight and yield by combining QTL pyramiding with speed breeding in Brassica napus L

Thousand-seed weight (TSW) is a critical target for genetic improvement in rapeseed ( Brassica napus L.). However, phenotypic selection for this trait remains challenging due to its polygenic regulation by multiple quantitative trait loci (QTL). Here, six favorable TSW QTL alleles from two donor parents were introgress into an elite restorer line, 621R, using an integrated strategy combining marker-assisted backcrossing and speed breeding protocols. Through six rounds of backcrossing and convergent crossing followed by two generations of selfing strategies, we developed 13 advanced lines with diverse TSW QTL combinations within 24 months. Field evaluations across three environments revealed that all lines exhibited significantly increased TSW in spring conditions (Minle, Gansu) and winter environments (Wuhan and Jiangling, Hubei) except for two lines which only showed increase in the spring environment. Hybridization assays using these lines as male parents crossed with two male-sterile lines (RG430A and 616A) demonstrated transgressive segregation for TSW: For RG430A-derived hybrids, all crosses significantly outperformed the original control (RG430A×621R) in Wuhan, with 8/13 and 9/13 crosses showing significant TSW increases in Minle and Jiangling, respectively. For 616A-derived hybrids, 11/13 and 10/13 crosses exhibited significant TSW enhancement in Minle and Jiangling, compared to 3/13 in Wuhan. Notably, two top-performing hybrids achieved 13.0% and 6.8% higher plot yields, respectively. Our results demonstrate that strategic pyramiding of complementary TSW QTL alleles effectively enhances seed weight in rapeseed, and these improved lines represent valuable genetic resources for developing high-yield hybrids.