| Narrow genetic basis has limited the improvement of Brassica napus L. (AACC, rapeseed). B. oleracea L. (CC,2n=18), one of the ancestral species of rapeseed containing several wild and cultivated forms, plays an important role in the improvement of rapeseed. In this study, a strategy of virtual allopolyploid was firstly proposed to investigate the genetic relationships between B. oleracea and the C subgenome of B. napus, and to evaluate the potential of B. oleracea on widening the genetic base of B. napus. Secondly, the resistance sources against Sclerotinia sclerotiorum were identified in Brassica crops especially in B. oleracea, and resistance QTLs were then mapped. Results were listed below.1. Genetic relationships between C subgenome of rapeseed and B. oleraceaAlthough there are a number of different allopolyploids in the plant kingdom, the exact ancestral parents of some allopolyploids have not been well characterized. We propose a strategy in which virtual allopolyploid lines derived from different types of parental species are used to investigate the progenitors of an allopolyploid. The genotypes of the parental lines and the natural allopolyploid were established using a set of DNA molecular markers. The genotypes of the virtual lines were then derived from those of the parental lines, and compared extensively with that of the natural allopolyploid. We applied this strategy to investigate the progenitors of the C subgenome of Brassica napus (rapeseed, AACC). Genetic structure was compared among natural rapeseed, virtual rapeseed lines, and their parental lines by principal component analysis and analysis of ancestry. Our data showed that the C subgenome of natural rapeseed was related closely to the genome of cultivated B. oleracea and its related wild types, such as B. incana, B. bourgeaui, B. montana, B. oleracea ssp. oleracea and B. cretica. This finding indicated that these types or their progeny might be ancestral donors of the C subgenome of rapeseed. On the other hand, several wild forms of B. oleracea such as B. rupestris, B. macrocarpa, B. villosa, B. insular is and B. hilarionis exhibit strong potential to widening germplasm of rapeseed.2. Screening for resistant resources against S. sclerotiorum in BrassicaStem rot caused by S. sclerotiorum is one of the most devastating diseases of rapeseed. Sources with high level of resistance are not available in rapeseed. In this study, plants in six Brassica species were evaluated for resistance to S. sclerotiorum in leaf and stem. Our data showed that,1) there was no significant difference (P=0.9161) for pathogenicity between S. sclerotiorum isolates CQ (from China) and GK (from Germany) when they were used to infect detached-leafs in Brassica crops,2) the optimal time to measure the lesion on leaf and stem was 3 and 10 days after inoculation respectively,3) High correlation (r=0.652,P<0.01) was found for resistance between leaf and stem; 4) among the six Brassica crops, B. oleracea had the highest level of resistance, followed by B. carinata, B. juncea, B. napus and B. nigra, whereas B. rapa had the lowest level of resistance, and 5) some wild types of B. oleracea such as B. rupestris, B. incana, B. insularis and B. villosa had excellent resistance against S. sclerotiorum.3. Mapping for resistance QTL against S. sclerotiorum in B. oleraceaA F2-clones population derived from a resistant accession of B. oleracea was developed by tissue culture, consisting of 149 lines with several clones for each line. The QTLs were mapped for resistance against S. sclerotiorum and resistance-related traits such as flowering time and epidermal hair. Results were shown as follows.1) The segregation of F2 is successive for relative resistance to Zhongyou 821 in leaf and stem, with average of 0.599 and 0.517. A significant and positive correlation of resistance was found between stem and leaf.2) Nine linkage groups were constructed with 128 SSR and 12 AFLP-RGA markers, spanning a total of 922 cM. The amount of markers in a linkage group ranged from 10 to 27, with an average marker distance of 6.59 cM.3) In total,12 QTL of resistance against S. sclerotiorum were identified in 2009 and 2010 with the CIM. For leaf resistance, three and five QTLs were identified in the two years, jointly accounting for 19.7% and 66.6% of the phenotypic variation respectively, with overlapping of one QTL in two years. For stem resistance, each of two QTLs were identified in 2009 and 2010, together explained 22.1% and 19.5% of the phenotypic variation, with one common QTL between two years. Among all the resistant QTLs, the confidence intervals of one leaf resistance QTL was overlapped with that of one stem resistance QTL.4) Among 12 QTLs, the resistant alleles of all QTLs were derived from the resistant parent, and the resistant alleles were dominant in relative to susceptible alleles in five QTLs.5) Significant negative correlations were tested between flowering time and resistance (r ranging from 0.222 to 0.391). Totally,8 flowering time QTL were identified in two year with CIM analysis, with one overlapped QTL in two years which had a overlapping confidence interval with one stem resistance QTL.6) One QTL for epidermal hair was identified, explaining 10.5% of the phenotypic variation without overlapping interval with any resistance QTLs. No significant correlation was found between epidermal hair and resistance in stem and leaf.Meanwhile, the undergoing work was introduced in brief. The resistance QTLs and the resistant mechanism against S. sclerotiorum in B. oleracea were discussed, and the strategy to utilize B. oleracea for improvement of resistance in rapeseed was proposed. This research will be helpful for improving rapeseed with B. oleracea in practice and theory.
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