Genetic Analysis of Heterosis for Yield and Yield Components in Rapeseed (Brassica Napus L.) by Quantitative Trait Locus Mapping

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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.

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HETEROSIS is the superior performance of F1 hybrids relative to the midparent value (MPV) or to the better parent. While the practical application of heterosis in plant breeding is quite successful in many crops through the development of hybrid varieties, the basic understanding of the phenomenon is not very advanced. Three main hypotheses exist to explain the genetic basis of heterosis: the dominance, overdominance, and epistasis hypotheses (Crow 1999; Goodnight 1999). The dominance hypothesis supposes that deleterious recessive alleles of one of the parents are complemented in the F1 hybrid by the dominant alleles of the other parent. The overdominance hypothesis states that the heterozygous combination of the alleles at a locus is superior to either of the two possible homozygous combinations. Epistasis assumes that epistatic interactions between different loci are the reason for heterosis.

Currently, results from quantitative genetic experiments favor the dominance hypothesis (Crow 1999). On the other hand, theoretical considerations and some observations indicate that epistasis plays a significant role in the expression of heterosis (Goodnight 1999). In addition, results of multimeric enzyme studies are apparent examples of true overdominance (Stuber 1999).

The extent of heterosis in rapeseed has been analyzed in a number of studies with widely varying results, depending on the materials used. In spring rapeseed hybrids an average high parent heterosis of 30% with a range of 20-50% was observed, while for winter rapeseed hybrids an average high parent heterosis of 50% was reported, ranging from 20 to 80% as reviewed by McVetty (1995). In a literature review Becker (1987) reported midparent heterosis values for yield in the range of 4-63% with average heterosis of 30 and 27% for winter and spring rapeseed, respectively.

QTL mapping has been increasingly used in recent years for studying heterosis. In maize Stuber et al. (1992) identified QTL for seven agronomic traits, including grain yield. The prevailing mode of action was overdominance. Testing all possible pairwise combinations ofmarkers linked to the mapped QTL, no epistasis was found. A number of other studies (Graham et al. 1997; Lu et al. 2003; Frascaroli et al. 2007) showed that a variety of effects ranging from partial to overdominance, including pseudo-overdominance, play a role in the determination of heterosis in maize, while epistasis showed no significant influence.

In riceXiao et al. (1995) concluded that dominance is the major causal factor of heterosis. No overdominance and epistasis was detected. These results are in disagreement with a series of studies on heterosis in rice by Yu et al. (1997), Li et al. (2001), Luo et al. (2001), and Mei et al. …