| Cabbage (Brassica oleraces var. capitata L.) originated from the Mediterranean coast was introduced into China Since 16th century, now it is one of the most important vegetable crops in our country. The production of Cabbage has reduced tremendously because of the prevalence of downy mildew economically and environmentally, and Downy mildew disease has being rising in different areas. Now screening the germplasm materials resistant to Downy mildew has become the major aim of Cabbage breeding. In recent years, with the deep research of molecular biology ceaselessly, Cabbage breeding for disease resistance with Marker-assisted Selection (MAS) has showed huge potential. At present some excellent inbred lines resistant to Downy mildew have been obtained. The traditional method of screening resistant materials by field inoculation is very complicated and laborious, even influenced by environment seriously in process of breeding for disease resistance and transfer. So, a sort of molecular markers linked to the resistant gene might be more effective, which can do in any growth and development period and improve Cabbage breeding for disease resistance efficiency greatly.In this study, we used resistant material 'R103' and susceptive material 'S101', we identified the pathogen which infected susceptive parent and analyze the resistance genetic rule, we used AFLP technique and the method of bulked segregation analysis (BSA) to study the F2 population which was constructed by the progeny of 'R103' and 'S101', 'S101' is a susceptible inbred line, 'R103' is an inbred line resistant to Downy mildew in field. Each individual of the F2 population was classified as resistant or susceptible by the method of visual observation and respectively. According to the result of identification, we chose the most resistant individuals and the most susceptible individuals to construct two pairs of different pools to screen the AFLP marker linked to the resistant gene in the Cabbage. Then convert AFLP marker into SCAR marker. The main results of the experiment were as fellows:(1) We identified the pathogen of susceptive material 'S101', by microscope detection and back inoculation experiment, Cabbage is infected by Hyaloperonospora parasitica strain. The practical ratio of resistant individuals to susceptible individuals was 2.93:1 in the F2 population, It is in accordance with the ratio of 3:1 by mean of the Chi-square test, and the population of BC1 with r susceptive back crossing parent is in accordance with the ratio 1:1, the population of BC1 with resistant back crossing parent shows total resistance. Resistance to Downy mildew of this population is controlled by a pair ofdominant gene.(2) Two pairs of different resistant and susceptible pools were constructed with the F2 population of the most resistant individuals and the most susceptible individuals. Total 64 pairs of primer combinations which consisted of 8 EcoR I primers and 8 Mse I primers were used to screen the AFLP marker by BSA method, As a result we found some differential fragments in 7 pairs, then identified two AFLP molecular markers linked to Downy mildew resistant gene by parents and another pool, which were amplified with the primer combination EcoR 1-AAG/Mse 1-CTC and EcoR 1-AAC/Afee 1-CAC, named Bo/?aag/ctc and 5o/?aac/cac-(3) 5o/?aag/ctc and BoRwckc two AFLP fragments were reconvered, cloned, and tranfered into E. coli DH5a, then extracted plasmid and sequenced. According to the result of two pairs of specific primers were designed. PCR results from Cabbage genomic DNA in parents , two pair pools and the F2 population showed that one fragment was amplified in resistant and susceptible population, different in quantity obviously, the other AFLP markers linked to the gene resistance to Downy mildew has been converted into a SCAR marker successfully, the exchange rate was 10%, and the map distances with the resistant gene was 7.8cM computed by the MAPMAKER/EXP (Version 3.0) map software, then called it BoRaag/cici 13-...
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