Genetic Analysis And QTL Mapping Of Days To Flowering And Photoperiod Sensitivity In Brassica Napus L.

In higher plant, flowering is one critical event during the whole growth stage andalso one significant mark in the transition from the basic vegetative phase (BVP) toreproductive phase (RP). Flowering early or late, or days to flowering (DTF) will produceimportant impacts on the maturity, harvesting time and yield of seed. To our bestknowledge, DTF is one quantitative trait with a complex genetic basis and regulated byendogenous flowering genes, hormone level, physiological and biochemical processesand environmental factors (mainly including photoperiod and temperature) together.However, for spring Brassica napus cultivars, photoperiod influences DTF moresignificantly than temperature does. Photoperiod sensitivity (PS) can limit the geographicadaptation of Brassica napus to a great extent. For example, Q2, used as one parent in thisstudy which is one spring and photoperiod-sensitivity Brassica napus, only can be plantedin the long-day region as a summer crop for ensuring maturity and yield of seed. Thegenetic control of DTF and PS has been studied extensively in the model plant rice andArabidopsis, and Brassica species, soybean, maize, sorghum, sunflower, oat and so on.But inheritance analysis and QTL mapping of PS in Brassica napus has not been reportedso far. In the present study, utilizing the microspore culture method doubled haploid (DH)lines were produced from one F₁ plant of the cross between two spring Brassica napuscultivars, Hyola401 and Q2, which exhibited a low and high sensitivity to photoperiod,respectively. P₁, P₂ and DH lines were together planted in three locations, one location(Hezheng)with a long and the other two (Wuhan and Zhaoqing) with a short photoperiodregime for consecutive two years. Inheritance analysis of DTF and PS in Brassica napuswas performed by using a mixed major and polygene genetic model. Additionally, usingQTLMapper 1.6 the total genetic component of DTF and PS was partitioned intomain-effect QTLs, epistatic QTLs and QTL-by-environment interactions (QEs) (only forDTF). The main results of this study are as follows:1. Three F₁ crosses of Hyola401×629jia, Hyola401×Q2 and Hyola401×Surpass400, and one F₂ from Hyola401 inbreeding were produced in the field of Wuhanand used to study the effect of genotype on the embryogenesis of microspore in Brassicanapus. Meanwhile, the effect of sampling time, pre-culture temperature, pre-culture timelength and the concentration of colchicine on the embryogenesis or doubling efficiencywas studied by using the F₁ cross of Hyola401×Q2. The results showed that there was ahighly significant difference (p＜0.01) among different genotypes for the embryogenesisof microspore and that the optimal sampling time, pre-culture temperature, pre-culturetime length and the concentration of colchicine were 7:30 AM, 32℃, 48 h and 50 mg/L,respectively. 2. The results implied that the best fitting genetic model for DTF at Hezheng, Wuhanand Zhaoqing was G-1, E-1-1 and E-1-6, respectively. In other words, the trait DTF in theDH population at Hezheng, Wuhan and Zhaoqing was mainly controlled by threeadditive-epistatic major plus polygene, two additive-epistatic major plus polygene andtwo duplicate major genes plus polygene, respectively. Heritability values of the majorgenes were 91.13%, 63.05% and 62.02%, and those of polygene were 4.43%, 1.58% and22.71%, respectively. For PSI, the best fitting genetic model was E-1-5. It wasconditioned by two epistatic-recessive major genes plus polygene. The heritability valuesof the major genes and polygene were 50% and 37.5%, respectively. Additionally, othergenetic parameters were also estimated (Table 19). Therefore, it is speculated that bothDTF and PS in this DH population were controlled by more than 2 major genes pluspolygene. Furthermore, the plants might have some different interaction models amongflowering genes under different photoperiods.3. Based on one DH population consisting of 149 DH lines which were randomlyselected from more than 600 DH lines one genetic linkage map of Brassica napus wasConstructed by MAPMAKER/EXP Vet.3.0. A total of 263 molecular markers showedstable and clear polymorphisms between two parents. Among them, there were 248molecular markers including 88 SSR markers, 101 SRAP markers and 74 AFLP markersdistributed on 19 linkage groups (LG), N1 to N19. The remaining 15 markers could notbe linked to any one LG. The total and average genetic distance between adjacent markerloci was 1634.7 cM and 6.6 cM, respectively. The percentage of segregation distortionmarkers mainly clustering on N4 and N5 was 27.6% (p＜0.01).4. Using QTLMapper1.6 we performed QTL mapping of DTF in this DH population.Three main-effect QTLs controlling DTF, dtf5, dtf11 and dtf18, were identified andtogether accounted for 58.3% of the phenotypic variation. Five pairs of loci including twomain-effect QTLs (dtf5 and dtf11) were involved in epistatic interaction and explained15.3% of the total phenotypic variation. Besides, among all of 12 pairs of QTLs, all ofthree main-effect QTLs and four pairs of epistatic QTLs interacted with environment. Inall, QEs accounted for 11.8% of the phenotypic variation.5. A total of four main-effect QTLs conditioning PS, ps3, ps10, ps14 and ps18, weredetected in this study and in all explained 53.3% of the phenotypic variation. In themeantime, six pairs of QTLs including two main-effect QTLs (ps3 and ps18) wereinvolved in epistasis and accounted for 27.6% of phenotypic variation. One main-effectQTL, dtf18 or ps18, located between one co-dominant SRAP marker em5-me9 and onedominant AFLP marker EA9-MG6, simultaneously controlled DTF and PS, andaccounted for the largest percentage of the phenotypic variation, 28.4% for DTF and 24.99% for PS. It can be used to improve rapeseed cultivars through marker-assistedselection (MAS).