A procedure to minimize the linkage drag in backcross breeding of rapeseed (Brassica napus) through irradiation

Backcross is an efficient breeding procedure in transferring traits governed by one or a few major genes. However, undesirable genomic segments linked to the target genes (so-called linkage drag) often prevent backcross projects' products from reaching practical utilization. This study aimed to explore approaches to minimize linkage drag in backcross breeding in rapeseed (Brassica napus). Plants of BC6F1, which were derived by crossing two rapeseed varieties, a double low x a double high (erucic acid, glucosinolate), were exposed to various doses of gamma rays (0, 15, 25, 35, 40, 50, 100, 150, 200, 250, 300, 350 and 400 Gy) in order to irradiate the young buds (2-3 mm in length). The effects of irradiation on the growth and development of the floral organs were investigated, and microspores were isolated and cultured in vitro to produce doubled haploids. Buds were seriously damaged and finally died on plants treated with 100 Gy and higher doses while those on plants treated with 50 Gy and lower doses survived at reduced flower size and delayed growth and development. Pollen viability from plants irradiated by 50 Gy and higher doses decreased (as assessed by staining), but the percentage from plants irradiated with 40 Gy and lower doses had a slight increase. Siliques developed on plants treated with 50 Gy and lower doses, were only 58-71% as long as control and seed number per silique dropped to 56-23% of the non-irradiated control. Severe chromosome damage and distortion were observed at various developmental stages in pollen mother cells from plants irradiated with 350 or 400 Gy gamma rays. Microspore-derived embryos were obtained from 40 Gy and lower doses treated plants but no embryo was obtained from plants irradiated with 50 Gy and higher doses. Leaf color and fertility mutants were identified among the progeny population derived from 50 Gy plants while the majority of them grew and developed normally. A few normal double low plants were identified in the populations. Two lines had a similar morphological performance as the recurrent parent but the yield differed significantly. One of them had the same performances in both seed yield and Sclerotinia sclerotiorum infection as the control. Thirteen out of 311 doubled haploid (DH) plants were recovered with double low quality and RFLP analysis with these lines is under way. (author)