Genetic analysis of heterosis for yield and yield components in rapeseed (Brassica napus L.) by quantitative trait locus mapping

Abstract

The main objective in this research was the genetic analysis of heterosis in rapeseed at the QTL level. A linkage map comprising 235 SSR and 144 AFLP markers covering 2045 cM was constructed in a doubled-haploid population from a cross between the cultivar "Express" and the resynthesized line "R53." In field experiments at four locations in Germany 250 doubled-haploid (DH) lines and their corresponding testcrosses with Express were evaluated for grain yield and three yield components. The heterosis ranged from 30% for grain yield to 0.7% for kernel weight. QTL were mapped using three different data sets, allowing the estimation of additive and dominance effects as well as digenic epistatic interactions. In total, 33 QTL were detected, of which 10 showed significant dominance effects. For grain yield, mainly complete dominance or overdominance was observed, whereas the other traits showed mainly partial dominance. A large number of epistatic interactions were detected. It was concluded that epistasis together with all levels of dominance from partial to overdominance is responsible for the expression of heterosis in rapeseed.

Figure 1.—

Genetic linkage map of B. napus from the cross “Express” × “R53.” The positions of the marker loci represent the distance from the first marker of the respective linkage group in centimorgans, estimated from the recombination frequencies between consecutive markers determined in 96 doubled-haploid lines. The recombination frequencies were transformed to centimorgans according to the Kosambi mapping function. The markers in bold italics were chosen for the construction of the framework map. The framework map positions, presented in parentheses, were estimated from recombination frequencies according to Haldane's mapping function on the basis of 250 DH lines. Markers deviating significantly from the expected 1:1 segregation ratio are labeled with “+” if skewed toward Express alleles and with “−” if the R53 allele is the more frequent one. The ovals, rectangles, and triangles indicate QTL identified in the doubled-haploid (DH) population and in the testcross (TC) population, and the midparent heterosis data, respectively. The different patterns illustrate QTL for grain yield (GY), thousand-kernel weight (TKW), seeds per silique (S/Sil), and siliques per square decimeter (Sil/dm²).