Molecular Dissection Of Genetic Bases Of Important Agronomic Traits In Oilseed Rape

Most of the important agronomic traits in crops are quantitative,and variations in these traits are usually controlled by many minor genes.Improvement of these quantitative traits is one of the major objectives in rapeseed genetic breeding program. Many elite varieties and hybrids in Brassica napus have been developed using traditional breeding methods,but the efficiency to breed them is still low because the genetic basis for most of the quantitative traits is still unclear.The advent of molecular markers,improved QTL mapping theory and novel experimental designs accelerated the dissection of quantitative traits in the last decade in B.napus.In this study,we employed two permanent populations of a highly heterotic B.napus cross,Quantum×No.2717-17.One population is 258 doubled haploid (DH) lines derived from the microspore culture of F₁,and the other one is the "immortalized F₂"(IF₂) population comprising 258 crosses resulted from randomly intermating of DH lines.Field experiments with 2 replicates following a randomized complete block design were implemented in three environments.Molecular markers were used to analyze the genetic model associated with 11 traits' performance and heterosis.The main results are summarized as follow:1.A genetic linkage map,spanning 1747.4 cM with an average interval of 4.4 cM,was constructed using 207 simple sequence repeat(SSR) markers and 190 sequence-related amplified polymorphism(SRAP) markers.Based on the public information of microsatellites(SSR markers),our map was aligned with reference maps.2.In the DH population,129 markers(32.5%) skewed from the expected 1:1 genotypic segregation by Chi-square test,and most of them skewed towards the Quantum(female parent).Compared to the DH population,36 more markers in the IF₂ population showed segregation distortion.3.In the DH population,the allele frequencies of Quantum and No.2717-17 in all 397 informative loci were 52.1%and 47.9%respectively.The allele ratio approximated to the expected 1:1.The genome and their proportions in the IF₂ population are similar to those in an F₂ population.The proportions of the genotypes for Quantum,No.2717-17 and heterzygousity are 27.08%,24.03%and 52.3%,respectively.4.Most traits in both populations showed transgressive segregation and demonstrated the normal distribution.There existed significant difference in all traits among three environments.The interactions between genotype and environment were observed for all traits.Transgressive segregation was observed for all traits' heterosis in the crosses of the IF₂ population,and the heterosis values of most traits displayed the normal distribution.5.The entire genome was searched for QTL conferring significant effects on all scored traits by composite interval mapping in two populations.The number of QTL varied according to the different traits.A total of 88 QTL were detected for yield and its component traits in three environments and of these 18 QTL were detected for more than once.There were 116 QTL identified related to other traits, of which 58 ones were repeatedly detected.6.QTL positive and negative additive effects were dispersed in both parents for all the traits.Most of QTL were located in QTL clusters.Some QTL were specifically detected only in one population.7.By composite interval mapping,44 heterosis loci were detected for yield and its component traits in the IF2 population,and five HL were repeatedly detected.For other traits,67 HL were detected,and only two were repeatedly detected. Compared to QTL mapping,the detection of HL was more sensitive to environmental influence.8.The proportion of variation explained by single HL was relatively small.Twenty HL were also detected in QTL analysis.Like QTL,some HL for different traits formed clusters on linkage groups.9.A total of 118 epistatic QTL(E-QTL) were detected for yield and its component traits in the three environments using composite interval mapping with mixed model,and 3 of them were repeatedly detected over environments.For other traits, 0 to 21 E-QTL were resolved for each one,and 6 common E-QTL were repeatedly detected over environments.The detection of E-QTL was more sensitive to environmental effect than that of main QTL.Some main QTL were invovled in interaction.10.All possible two-way ANOVAs between the markers on whole linkage groups were analyzed for 11 traits in the IF₂ population.A large number of significant digenic interactions were detected for all traits.In a part of loci,two or more interaction types were detected in one pair of loci.All 4 possible digenic significant interaction types(AA,AD,DA,DD) were detected,and of them AA type occurred at predominantly high frequencies with DD type being the least frequent.For two-locus combinations of significant digenic interactions, two-locus homozygotes were the best genotypes in many cases,at the same time, another two-locus homologues combinations was also the worst genotype;On the other hand,double heterozygote were rarely the best genotype and never the worst genotype.Most interactions occurred between two loci both showing non-significant effects to traits.The effects of some QTL were influenced by epistasis.Pleiotropic effects of digenic interactions were detected between or among the related traits.11.The digenic interactions detected in the DH and IF₂ populations showed little overlapping,which suggested that effects of epistasis were influenced by the genetic background.