A comprehensive and precise set of intervarietal substitution lines to identify candidate genes and quantitative trait loci in oilseed rape (Brassica napus L.)

A set of intervarietal substitution lines were developed in rapeseed by recurrent backcrossing and marker-assisted selection and employed for mapping both qualitative and quantitative traits. Intervarietal substitution lines (ISLs) may be assembled into advanced secondary mapping populations that have remarkable potential for resolving trait loci and mapping candidate genes. To facilitate the identification of important genes in oilseed rape (canola, Brassica napus), we developed 89 ISLs using an elite cultivar 'Zhongyou 821' (ZY821) as the recipient and a re-synthesized line 'No.2127' as the donor. In the whole process of ISLs development, the target chromosome segments were selected based on the genotypes of 300 microsatellite markers evenly distributed across the genome. Eighty-nine ISLs fixed at BC5F4 were genotyped by sequencing using double digestion to survey the lengths of target substitution segments from the donor parent and the background segments from the recurrent parent. The total length of the substituted chromosome segments was 3030.27 Mb, representing 3.56 x of the Darmor-bzh reference genome sequence (version 4.1). Gene mapping was conducted for two qualitative traits, flower colour and seed-coat colour, and nine quantitative traits including yield- and quality-related traits, with 19 QTLs identified for the latter. Overlapping substitution segments were identified for flower colour and seed-coat colour loci, as well as for QTLs consistently detected in 2 or 3 years. These results demonstrate the value of these ISLs for locus resolution and subsequent cloning, targeted mutation or editing of genes controlling important traits in oilseed rape.