Genetic Analysis of Heterosis in Rapeseed (B. napus L.) by QTL Mapping

Heterosis, or hybrid vigour, refers to the phenomenon that progeny of diverse inbred lines exhibit greater biomass, speed of development and fertility than the better of the two parents or the midparent value between them. 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. In the current study a doubled haploid population of 250 lines and their corresponding testcross hybrids were grown and analysed in greenhouse and field experiments at four locations in order to generate phenotypic data for studying heterosis in rapeseed on the QTL level. The main objectives included:1. Identification of the levels of heterosis for agronomic important traits2. Identification, localization and determination of the effects of QTL for heterotic traits (grain yield and yield components, early plant biomass, plant height), phenological traits (beginning of flowering, end of flowering and duration of flowering) and quality traits (oil, protein, glucosinolate, erucic acid and sinapine content). 3. Assessment of the contributions of different genetic effects, e.g. dominance, overdominance and epistasis to the expression of heterosis in rapeseed4. Study of the correlation between molecular marker heterozygosity and hybrid performance5. Identification of hot spots for QTL involved in heterosisTo elucidate the genetic basis of heterosis in rapeseed QTL analysis was performed with three separate datasets, each of which provided information for different genetic effects. As a prerequisite for the QTL mapping a new genetic linkage map was constructed based on 250 doubled haploid lines developed from microspores of an F1 hybrid of a highly heterotic cross between the winter cultivar Express and a resynthesized line, R53 . The datasets used for QTL mapping included data from the doubled haploid population itself, a testcross hybrid population developed from crosses of the doubled haploid lines with the male sterile tester MSL-Express , and the midparent heterosis between the doubled haploid lines and their corresponding testcrosses. Using the three different datasets, the additive and dominance effects of the loci controlling the studied traits, as well as epistatic interactions contributing to trait variation could be estimated. The phenotypic data used for QTL mapping was derived from a field trial carried out in one year at four locations following alpha lettice design of the type 26 x 10. The 250 doubled haploid lines and their corresponding testcross hybrids were grown in different beds on the field and each doubled haploid line was followed by its corresponding testcross hybrid, which allowed the two genotypes to be grown in as similar conditions as possible, excluding the competition between the lines and the more vigorous hybrids. Out of 14 traits studied, the highest levels of heterosis were observed for early fresh biomass and total grain yield, indicating that more complex traits show higher heterotis. Considering 9 heterotic traits (early fresh biomass, grain yield, thousand kernel weight, seeds per silique, siliques per square decimeter, plant height, beginning of flowering, end of flowering, and duration of flowering) 60 QTL with additive effects were identified using data of the doubled haploid population. Twenty seven of them were congruent with loci exhibiting dominance effects in the testcrosses, which allowed the assessment of the degree of dominance. Of the 27 QTL 11 showed partial dominance, 2 full dominance, and 14 exhibited overdominance. From 37 QTL mapped for 5 seed quality traits (oil, protein, glucosinolate, erucic acid, and sinapine content) showing very low or no heterosis only 3 QTL demonstrated overdominance, while from the remaining 8 QTL with dominance effects one showed full dominance and 7 exhibited partial dominance. QTL mapping results for erucic acid and glucosinolate content indicated that the lack of heterosis at population level does not exclude the existence of dominance at locus level. For these two traits loci were observed with positive and negative dominance effects, whose absolute values were of similar magnitude. Most probably the cancelling of the dominance effects with opposite signs led to the lack of heterosis at population level. The QTL mapping for loci involved in epistasis resulted in the localisation of a total number of 99, 112 and 101 epistatic locus pairs mapped with doubled haploid line, midparent heterosis, and testcross hybrid data, respectively. Most of the digenic interactions (85%) were between loci showing no significant main effects. The results of the current study indicated that all levels of dominance in the range from partial dominance to overdominance play a role in the expression of heterosis in the rapeseed population under study. The large number of epistatic interactions observed showed that epistasis also contributes to heterosis in rapeseed, often explaining as large or a larger portion of the phenotypic variance than the main effect QTL. In general the degree of correlation between genome heterozygosity and phenotypic traits reflects the importance of overall genome heterozygosity to trait expression. In the current study no correlation was observed between the overall genome heterozygosity and heterosis. The lack of correlation and the QTL mapping results demonstrated that there exist specific loci, often showing overdominance, which are responsible for the expression of heterosis, meaning that the heterozygous status of these particular loci is much more important for the increased hybrid vigour than the overall genome heterozygosity. Three regions in the rapeseed genome on linkage groups N11, N12, and N19, were identified as hot spots for QTL as an apparent clustering of QTL with additive and dominance effects was observed at these sites. The three clusters included QTL for early fresh biomass, grain yield, thousand kernel weight, seeds per silique, siliques per square decimeter, plant height, beginning of flowering, end of flowering, duration of flowering, oil content, and glucosinolate content, mapped with at least one of the three datasets. Despite the extreme genetic complexity of the studied traits, with numerous epistatic interactions influencing heterosis, which would impede marker assisted selection (MAS), some major QTL with relatively large dominance effects and the three hot spots for heterotic QTL could be of further interest for practical breeding. With the help of MAS the alleles of QTL showing overdominance could be purposefully distributed between complementary heterotic gene pools in order to benefit maximally the heterosis in F1 hybrids derived from crosses between these pools. By identifying QTL involved in heterosis the current study for the first time provided information on the contribution of different genetic effects, e.g. partial, full, overdominance, and epistasis to the expression of heterosis in rapeseed. QTL mapping in a population derived of a single cross is only the first step in the molecular dissection of a quantitative trait. A second step should include a verification of the current results in other populations. Further, fine mapping of selected QTL with strong effects on heterosis could be carried out in advanced backcross populations developed with the help of MAS.