| Brassica napus is one of the most important source of vegetable oils. Developing early maturiting rapeseed varieties are important for effectively using farmland and avoiding the unfavorable environments. Flowering time of rapeseed could directly affect its maturity stage and yield level. However, it is a complex quantitative trait, controlled by a number of genes and influnced by environments. Many efforts have been made to explore its genetic control system, which largelly promote the breeding programs for early maturing rapeseed breeding.In this study,282 DH lines were used, which was produced in previous study from across between the German winter rapeseed cultivar’Sollux’and the Chinese variety’Gaoyou’. Two parents differ in flowering time around one month. Besides, based on the primary QTL study under nine environments, we aimed to isolate and fine mapping the three most important QTL qFTA2, qFTC2 and qFT6 by constructing BC2F1 backcross populations. Futher, we analysed the genetic relationships between flowering time and seed weight by conditional QTL approach. The main results obtained are as follows:1. Phenotypic variation of flowering time in 9 growing environments:The difference of the average flowering time between the two parents was 26 days. The number of days from sowing to flowering time showed a continuous variation in all 9 environments, which was a typical quantitative trait feature. There were no obvious transgressive segregations among 282 DH lines in each environment, indicating that the control of the late flowering genes mainly derived from Sollux, and the early genes were from Gaoyou.2. QTLidentification of flowering time and their genetic effects:seven major QTLs, which showed significant additive effects minimum in three environments. The additive effects ranged from 0.58-3.85 days and the LOD values were 2.76-30.2. The average contribution rates were 3.51-19.13. Together the additive effects of all seven QTLs accounted for around 84% of the phenotypic variation in population, while the sum of epistatic effect from the eight pairs of loci was around 41.8% of the total effect. QTL xenvironment interactions were small and only significant in few observation locations.3. The genetic influence of flowering time on the 1000-seeds weight:The significant negative correlation was found between the flowering time and grain weight.The conditional QTL analysis revealed large impact of flowering time on seed weight in four QTLs qSWA2, qSWA3, qSWA4 and qSWC2.This partly explained the significant negative correlation between these two traits. However, the most important two QTLs for seed weight qSWA 7 and qSWC8 did not show correspondence to flowering time.4. Integration of molecular markers within three major QTL regions:On the basis of the initial QTL analysis of flowering time, we focused on the three major QTLs qFTA2, qFTC2 and qFTC6 for further fine mapping. Firstly, by the means of sequence information from Brassica rape, Brassica oleracia and Brassica napus, a large number of locus specific markers were designed. Finally,10 new markers were added into the qFTA2 region (ZAAS408b—SUC1-3), the average genetic distance between the two markers was narrowed from 3.1cM to 1.38cM.There were 30 new markers assigned into the qFTC2 interval (CN32a—ZAAS551),the average genetic distance between the markers was reduced from 3.88cM to 0.58cM. The makers in qFTC6 interval (ZAAS763—ZAASA7-47) increased from 8 to 18, the average genetic distance between the markers was reduced from 2.92cM to 1.21cM.5. Developing backcross populations and fine mapping qFTC2and qFTC6:To fine mapping qFTC2 andqFTC6, we developed BC2F1 populations by marker assistant selection. Meanwhile, combining the marker genotypes and their corresponding phenotypes of flowering time, the substitution mapping was performed. As the results, the qFTC2 interval was reduced from the original 20.5cM to about 2.8cM, corresponding to a physical distance of brassica napus about 2.87Mb, the difference of flowering time between heterozygous and homozygous alleles was 3.22 days, which showed consistent with the initial QTL analysis (a=2.93 days). The QTL region of qFTC6 was narrowed to about 13.1cM from the original 23.4cM, corresponding to the physical map distance of about 1.5Mb. The difference of flowering time between heterozygous (SG) and homozygous Sollux alleles (SS) was 2.14 days, and the difference between heterozygous (SG) and homozygous Gaoyou alleles (GG) in average was 3.44 days, which were also basically agreement with the initial qFTC6 additive effect (a=2.02day).6. Marker based selection for breeding purpose:20 latest and 20 earliest flowering time lines and six co-dominant markers from the peak positions of three major QTLs (qFTA2, qFTC2 and qFTC6) were screened out for association analysis. The result showed a good fitness between marker genotypes and trait phenotypes (70-100%), indicating their potentials for breeding purpose to transfer the early maturity genes into breeding materials. Furthermore, results showed that there were significant difference of 1000-seeds weight (0.328g) between the two earliest and the later flowering groups (P= 0.015), but the oil content and seeds number per silique had no significant difference (0.274 and 0.189) between two groups.Thus, these markers linked to the 3 major QTL were of great potentials in breeding programms. |
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