| Chinese cabbage (Brassica rapa L. ssp. pekinensis) originated from China is one of the most important vegetable crops in Asia, especially in China, Japan, and Korea. Since the 1950s, the production of Chinese cabbage has reduced tremendously because of the prevalence of virus disease in the Chinese cabbage. Virus disease has become one of the third most destructive diseases of Chinese cabbage. Turnip mosaic virus (TuMV) which has several strains is the most important virus disease in Chinese cabbage in terms of crop damage. In China, Chinese cabbage is infected by a mixture of strains.TuMV-C4 is the major strain which make up about 45.2% of all isolates of this virus. However TuMV-C5 was the most virulent stain. Now screening the germplasm materials resistant to TuMV has become the major aim of Chinese cabbage breeding. And some excellent inbred lines resistant to TuMV have been obtained. The traditional method of screening resistant materials by field inoculation is very complicated and laborious. A sort of molecular markers linked to the resistant gene might be more effective.APLP (amplified fragment length polymorphism) is a powerful DNA fingerprinting technique. In this study, we used AFLP technique and the method of bulked segregation analysis (BSA) to study the F2 population which was constructed by the progeny of B4p058+Brpl08, Brp181 is a susceptible inbred line, Brp058 is an inbred line resistant to TuMV. After inoculation with TuMV-C5 strain, each individual of the F2 population was classified as resistant or susceptible by the method of visual observation and enzyme-linked immunosorbent assay (ELISA) respectively. According to the result of identification, we chose the most resistant individuals and the most susceptible individuals to construct three pairs of different pools to screen the AFLP marker linked to the susceptible gene in the Chinese cabbage. The main results of the experiment were as fellows:1. The practical ratio of resistant individuals to susceptible individuals was 1:2.87 in the p2 population , It is in accordance with the ratio of 1:3 by mean of the Chi-square(x2) test. Resistance to TuMV-C5 in this population is controlled by a pair of recessive gene.2. Three pairs of different resistant pools and susceptible pools were constructed with the preamplification product of the most resistant individuals and the most susceptible individuals. Total 128 pairs of primer combinations which consisted of 8 EcoRI primers and 16 Msel primers were used to screen the AFLP marker by BSA method, As a result we identifed two AFLP molecular markers linked to TuMV susceptible gene with the exchange frequency of 7.76% and 6.90% respectively which were amplified with the primer combination EcoRI-AG/MseI-CAC and EcoRI-AG/MseI-GAC.The sizes of both markers are 150bp. The map distances were 8.4cM and 7.5cM respectively computed by the MAPMAKER/EXP (Version 3.0) map software.They were named CAC150 and CAG150.3. One of the AFLP markers linked to the gene susceptible to TuMV has been converted into a SCAR marker successfully, after two AFLP fragments were recovered, cloned and sequenced. According to the result of sequencing two pairs of specific primers were designed. PCR results from Chinese cabbage genomic DNA in the F2 population showed that only one special band was amplified with the primer designed according to the sequence of CAC150, and the map distance of SCAR marker is 16.06cM. |
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