Genetic Studies And QTL Mapping Of Seed Vivipary In Brassica Napus L

The rapeseed is one of the most important oilcrops with high economic status in China. Great progress has been made in the research and application of heterosis in rapeseed after the successful application of heterosis in rice. In 2000-2005, 217 rapeseed cultivars had been approved in China, consisting of 47 open-pollinated cultivars and 170 hybrid cultivars (Zhou and Fu., 2007; 12th IRC). To 2005, the hybrid cultivars promotion area accounted for 71.23% of the total rapeseed planting area, amounting to 5,600,000 hectares. However, seed vivipary sometimes is a problem in rapeseed, especially in production of commercial F₁ hybrid. It not only affected the quality of seed, but also affected the yield. At present, the research on seed vivipary in rapeseed mostly focused on physiology, characteristics and influence on the seed quality, not on the deeply dissection. In this study, 4 vivipary-susceptible lines and 2 vivipary-tolerant lines were selected from many rapeseed inbred lines, and a preliminary study on dormancy in rapeseed has been done; and many populations have been created for genetic and QTL analysis. The main results were as following:1. The viviparous percentage had been investigated in 27 rapeseed lines under field condition. There was a great difference on viviparous percentage in different rapeseed lines. Four vivipary-susceptive lines (7601, 7605, 7615 and 7622) and two vivipary-tolerant lines (2411 and 117AB) were selected out. The difference of viviparous percentage for two years and two locations indicated that environment factor plays an important role. The viviparous percentage under wet sand wasn't consist with the viviparous percentage under field condition. This showed that it is different in the shell combination degree and inhibitory substance.2. The vivipary occurrence time of 7605, 117AB and F₁ (7605×117AB) were observed and found that the vivipary of 7605, 117AB and F₁ occurred at 22 days, 34 days and 31 days after flowering, respectively. The viviparous percentage of 117AB and F₁ was positively related to the number of days after flowering and the viviparous percentage of 7605 was significant positively related to the number of days after flowering. 3. The germination test of four vivipary-tolerant lines (2409, 2410, 2411 and 117AB) had been done and the germination percentage of 2409, 2410 and 2411 exceeded 80% at the second day, but the germination percentage of 117AB was 34.7% after 7 days. So117AB was a slightly dormant line.4. Endogenous hormones content of pods of 7605, 117AB and F₁ (GA3, IAA, ABA and Z) were determined by HPLC at 25 days, 35 days and 45 days after flowering, respectively. The IAA in 7605 was highest at the initial stage of seed vivipary, so IAA could be related to the initiation of seed vivipary. The GA3 content in 7605 was highest at 35 days after flowering and ABA content was lowest at the same time; so the GA3/ABA was highest at 35 days after flowering and it was consistent with the degree of vivipary in 7605. This indicated that the GA3 content was proportional to the seed vivipary percentage and ABA content was in inverse proportion to the seed vivipary.5. The back and transverse section of seed coat (7605,117AB and F₁) were observed by scanning electron microscope and found that the structure of the seed coat between 117AB and 7605 are different. The out integument of 117AB is thicker than that of 7605 and the back of 117AB is compacter than that of 7605. The stomatal number of 117AB is less than that of 7605.6. The seed vivipary of the cross 7605×117AB was controlled by one major gene with additive effects plus polygene with additive-dominance effects (the D-2 model) by jointly segregating analysis. The additive effects and dominance effects of major gene were estimated as 10.1121 and 0, while those of polygene were 14.1949 and -13.8547, the degree of dominance was 0.9760 showing partial dominance. Heritability of the major gene of F₂ and F_2:3 populations were estimated as 54.9739% and 46.2356%, while those of polygene were 12.6633% and 8.2116%, respectively.7. 413 polymorphic molecular markers were found in the F₂ population. Eliminating 53 distorted markers, 360 molecular markers were used to construct the linkage map and 277 markers were distributed on 21 linkage groups (LG). Based on the public information of microsatellites (SSR markers), our map was aligned with reference maps. The linkage map covered 2667.7cM totally, with an average distance of 9.6cM, and can be used for QTL mapping. 8. Five QTLs for seed vivipary were detected by CIM (composite interval mapping, LOD≥2.5) and located on LG2 and LG7, respectively. They were named as qPHS-2-a, qPHS-2-b, qPHS-2-c, qPHS-2-d and qPHS-7 and located in the vicinity of A3G12700, A13C1500, P3C4180, T2G13420 and P1G8300 respectively. Phenotypic variance explained by each QTL ranged from 4.11% to 50.78% and the five putative QTLs explained about 75.63% of the total phenotypic variance.9. A major QTL, qPHS-2-c, was identified on LG2 (N11) flanked by P3C4180 and C6C13160, which explained 50.78% of the total phenotypic variance. Four QTLs were located on LG2 and formed a QTL cluster. Three QTLs, qPHS-2-a, qPHS-2-b and qPHS-2-c, had additive model and the other two QTLs, qPHS-2-d and qPHS-7, had partial dominant model. This indicated that additive effects is predominant.10. The potential markers linked to seed vivipary have been detected for all polymorphic markers by One-way ANOVA. A few new markers were detected on LG6, LG12, LG13, LG15, LG19 and LG21, except for LG2 and LG7. Eight unmapped markers linked to seed vivipary were detected in this study.11. Four pairs of significant digenic epistatic interactions for seed vivipary were detected. Phenotypic variance explained by each epistatic interactions ranged from 2.09% to 9.40% and four pairs of epistatic interactions explained about 17.16% of the total phenotypic variance. One significant epistatic interaction between intervals P1G5370-P1T7310 and BRMS001-P1T7280 accounted for 9.40% of total variance. One pair of epistatic interaction was detected between the QTL qPHS-7 and the interval P1T5100-A14C330 and no epistatic interaction related to the major QTL.