| Sclerotinia stem rot (white mold), caused by fungal pathogen Sclerotinia sclerotiorum is a major constraint to production worldwide in Brassica napus L. (AACC, rapeseed). Yield loss ranges from 10% to 30% in China. There are no rapeseed varieties with complete resistance or immune to Sclerotinia. So it is urgengt to find resistance resources and develop resistant varieties of rapeseed. In addition, narrow genetic basis has limited the improvement of rapeseed. In previous studies we found that B. oleracea L. (CC,2n= 18), one of the ancestral species of rapeseed containing several wild and cultivated forms, has a high level resistance to S.sclerotiorum. In this study, a few accessions of B. oleracea were crossed with Zhongshuang 9 to develop hexaploid (AACCCC) lines via embryo rescue and colchicine chromosome doubling. And elite individuals were screened in the S4 of hexaploid (D5) derived from C01 with high level of resistance against Sclerotinia. The genetic rule of resistance was investigated among hybrid between D5 and rapeseed lines.1. Development of hexaploidBrassica napus Zhongshuang 9 as female was crossed with 39 different kinds of B. oleracea as male. Among 21 embryos germinated,18 embryos were identified as true hybrids in morphology, pollen fertility, cytology and molecular markers. But 11 were identified as hexaploid (AACCCC).2. Fertility and crossability of hexaploidThe fertility of 11 hexaploids and crossability between them and Zhongshuang 9 and B. rapa 5W397 were investigated. Significant differences (P< 0.01) were found among 11 hexaploids for seed-set in self-pollination and open-pollination, while high and significant correlation (r= 0.69, P< 0.01) was found for seed-set between self-pollination and open-pollination. The seed-set of hexaploids ranged from 6.3 to 26.4 seed per pod in self-pollination, from 8.1 to 31.1 seed per pod in open-pollination, respectively.ANOVA showed significant difference (P< 0.01) among crosses between hexaploids and B. napus, and between hexaploids and B. rapa. The crossability averaged 17.5 seeds per pod, ranging from 12.2 to 24.7 in the crosses between hexaploids and Zhongshuang 9, while the crossability ranged from 4.3 to 24.7 seeds per pod with average of 11.8 seeds per pod in the crosses between hexaploids and 5W397.3. Selection of sclerotinia resistance in the progenies of hexaploidResistance selection to sclerotinia was screened in the progenies of hexaploid derived from Zhongshuang 9 and C01, a wild B. oleracea line with high level of resistance. Significant difference among hexaploid individuals for both resistance and seed pod. After selection by evaluating resistance and screening with molecular markers linked with resistance, the resistance and seed set gradually increased from S0-S4.4. Genetic rule of resistance among pentaploid hybrids between B. napus and hexaploidTo investigate genetic rule of sclerotinia resistance, the hexaploid (D5-2) was crossed with nine lines of B. napus. It was easy to develop hybridization between hexaploid and B. napus. The pollen fertility was above 95% on average, while high and significant correlation between self-fertility and free-fertility was found in pentaploids (r=0.67, P< 0.05). The sclerotinia resistance of hybrids were between two parents and significantly associated with that of parental line of B. napus (r= 0.78,P< 0.05), indicating additive effects on Sclerotinia resistance in the pentaploid hybrid. |
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