| Rapeseed(Brassica napus, AACC, 2n = 38) is a worldwide major oil and economic crop with excellent characteristics, such as high yield and disease resistance. B. napus is an allotetraploid derived from naturally happened hybridization between B. rapa(genome AA, 2n = 20) and B. oleracea(genome AA, 2n = 18), followed by chromosome doubling. Compared with the parental species, the narrow genetic basis has limited the improvement of B. napus. Based on this, we proposed a new strategy to develop a new type B. napus carrying exogenous genomic components from B. oleracea via crossing B. oleracea with hexaploid(AAAACC, 2n = 58) that derived from B. napus and B. rapa. Meanwhile, with molecular marker-assisted backcross, we introgressed resistance locus from a wild species C01(B. incana), with a strong resistance to Sclerotinia sclerotiorum, into a cultivated B. oleracea var. alboglabra. Next, we hope to create the new type B. napus by using the hexaploid as a bridge. In the present study, the above-mentioned strategy was explored and verified. The results are as below.1. Development of allohexaploidTriploid with genome AAC was created via crossing B. napus Zhongshuang11(female) with B. rapa SWUA01(male). A total of 54 S0 plants were obtained after chromosome doubling with the treatment of 0.1mg/ml colchicines. Through morphology identification, pollen fertility checking, molecular detection and cytology investigation, seven lines which exhibited big buds and flowers, and had good pollen fertility and 58 chromosomes were identified as true allohexaploids AAAACC.2. Fertility of allohexaploidThe fertility of two hexaploidy S0 lines were investigated. The seed set of the two hexaploids ranged from 8.8 to 9.8 seeds per pod in self-pollination, and from 3.9 to 4.3 seeds per pod in open-pollination. General seed quality was observed in hexaploid, as the contents of erucic acid and glucosinolate were both intermediated between Zhongshuang 9 and Zhongyou 821.The S1 had a wide phenotypic variation. The self-pollinated seed set of S1 individuals ranged from 2.9 to 10.0 seeds per pod, with an average of 5.9 ± 3.0 seeds per pod, and open-pollinated seed-set ranged from 4.2 to 9.1 seeds per pod, with an average of 6.3 ± 2.8 seeds per pod. Significant differences were found among S1 individuals for both self- and open- pollinated seed set(P < 0.0001). Though significant difference was found between self- and open- pollinated seed set(P = 0.0345), a positive correlation(r = 0.39, P < 0.05)was found for seed-set between self-pollination and open-pollination.3. Introgression of resistance locus by molecular marker-assisted backcrossBC1, BC2 and BC3 were developed by the crossing a susceptible cultivated B. oleracea accession C41(B. oleracea var. alboglabra, as recurrent parent) with a wild B. oleracea accession C01(B. incana) which has a strong resistance to S. sclerotiorum. Since a major resistance QTL was previously detected on chromosome C09 from B. incana C01, we developed new SSR markers in the target QTL region, and successfully encrypted five markers into the QTL area. By marker-assisted selection and resistance evaluation, the BC3F2 was constructed and identified to be with the C09-resistance locus and the resistance against S. sclerotiorum derived from B. incana.4. Crossability of allohexaploidThe crossability between hexaploid S0 and the backcrossed offspring developed from ?C01‘ was investigated. We found that no seeds were available when using hexaploid S0 as female, while 0.05 to 0.1 seeds/pod was developed when using B. oleracea as the female. It indicates that it is possible to develop hybrid seeds under field condition by using B. oleracea as the female and hexaploid as male. Similarly, the crossiability between hexaploid S0 and B. rapa was 0.05 to 0.18 seeds/pod. In contrast, the crossability between hexaploid S0 and B. napus was much higher, with seed set ranging from 1.53 to 3.76 seeds per pod. Wide genetic variations and good fertility were found among the hybrids between hexaploid S0 and B. napus. These results suggested that it is possible to develop new-type and resistant B. napus through hybridization between the allohexaploid and the resistance backcrossed offspring of the wild B. oleracea which is resistant to S. sclerotiorum. Moreover, the cross between allohexaploid and B. napus was also useful in widening the genetic background of B. napus.The crossability between hexaploid S1 and various types of B. oleracea was investigated as well. The seed-set was about 0.05 seeds per pod when using hexaploid S1 as female, without significant difference among individuals(P = 0.1032). When using B. oleracea as female, the seed-set was about 0.03 seeds per pod. This suggests that it is possible to get hybrids between hexaploid S1 and B. oleracea under field condition, no matter which type of B. oleracea is used. The crossability of hexaploid S1 with B. rapa and B. napus were about 0.06 and 0.15 seeds per pod, respectively. These results indicate that, it is possible to create new-type B. napus by using the hexaploids as a bridge, and this will be helpful to widen the genetic basis of rapeseed and to improve rapeseed by introgressing genetic components from B. oleracea.Furthermore, the future work was introduced in brief, and a few aspects regarding using B. oleracea in rapeseed breeding were discussed, including the utilization of distant hybridization and polyploid breeding in rapeseed breeding, the stability and crossability of allopolyploid, and the selection efficiency in the backcrossed offspring. |
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