Construction of chromosome segment substitution lines of Chinese cabbage (Brassica rapa L. ssp pekinensis) in the background of RcBr (B. rapa L. ssp dichotoma) and characterization of segments representing the bolting trait

Chromosome segment substitution lines (CSSLs) are useful tools for precise mapping of quantitative trait loci (QTLs) and evaluation of gene actions or interactions in theoretical studies. To facilitate the genetic analysis of complex traits and use of marker-assisted breeding in Brassica rapa, we developed a set of CSSLs using the extremely early-flowering inbred line RcBr as the recipient and extremely late-bolting Chinese cabbage variety 08A061 as the donor parent. The CSSL population consisted of 120 lines containing a total of 275 substituted segments, of which 47 lines contained only one substituted segment. The total length of the substituted segments spanned 2489.75 cM with an average length of 9.05 cM, representing 91.0% of the B. rapa genome and each line consisting of an average 98.1% of the recurrent parent genome. A total of 15 putative additive QTLs and 10 pairwised epistatic QTLs for three bolting indices were subsequently detected by QTL IciMapping 3.2 under two growth environments. Phenotypic variation explained by the additive QTLs, with the total variation ranging from 3.31 to 31.29% and 10.04 to 75.83%, respectively. Overall, the epistatic QTLs explained only a small amount of the phenotypic variation. The CSSLs and analyses presented here will help enhance our understanding of the genetics of bolting in B. rapa, thereby facilitating future molecular breeding programs.