Identification And Genetic Mapping Of Tumv-resistant Gene In Brassica Rapa L. Ssp. Chinensis

Pak-cho(iBrassica rapa L. ssp. chinensis Makino)is a very important economical and nutritional vegetable crop in Asia and it accounts for the largest vegetable production area (30-40%) in China. It originated in China and began to grow at about 5th century AD. There are very abundant germplasm resources of pak-choi in China. Pak-choi (ssp.chinensis) and Chinese cabbage (ssp. pekinensis) are the most important crops of B. rapa and the genetic relationship between them is very close. Though the former has a longer history than the latter for several hundred years, the breeding programs for MAS of pak-choi are fewer than Chinese cabbage throughout the world.Turnip mosaic virus (TuMV) is one of the most important pathogens of pak-choi and causes serious losses. It can make the host plants mosaic leaves or necrosis and seriously affect plant photosynthesis. The natural plant resistance is the most effective method to control TuMV. TuMV resistance in Brassica and a number of resistance genes or QTLs have been reported around the world. But no molecular marker linked to TuMV resistance gene has been reported in pak-choi.In this study, Q048, a TuMV-resistant lines was crossed with A168-5D, a susceptible line. The F₂ (180 individuals) population was constructed and used to screen the moleular marker linked to the TuMV resistance gene and to construct the genetic linkage map of pak-choi.The seedlings of parents, F₁ and 180 F₂ individuals were mechanically inoculated TuMV-Hu1 (TuMV-C5). The resistance evaluation was done by ELISA kit and the data of resistant and susceptible phenotypes of F₂ population (145 resistant and 35 susceptible) were analysed by chi-square test, which is very close to the expected segregation of 3:1 with significant (Χ² = 2.96 <Χ²_0.05). AFLP technique and BSA method were used to study the F₂ population and the parents, F₁, resistant bulk, susceptible bulk, 10 resistant and 10 susceptible F₂ individuals were screened. One of the polymorphism bands amplified by EaccMctt was scored and the ratio of 132 presents to 48 absents is very close to the expected segregation of 3:1 with significant (Χ² = 0.27<Χ²_0.05). F₂ hybrids segregated for TuMV resistance in a 3:1 ratio is for goodness of fit the expected Mendelian model based on the action of a single dominant gene. 36 AFLP polymorphic primer combinations were employed to amplify the parents and F₂ population. The data indicated that the TuMV resistance locus was positioned between AFLP markers EaccMctt3 (7.8cM) and EatcMcac1 (20.3 cM). The resistance gene was named TuRBCH01 (TuMV resistance in Brassica rapa ssp. chinensis 01).A genetic linkage map for pak-choi was constructed using AFLP and SSR markers based on 180 F₂ individuals. It included 133 AFLP and 74 SSR markers from 57 AFLP and 65 SSR primer combinations, respectively. According to the reference genetic linkage map of Brassica rapa genome, 50 SSR markers were previously used to construct a framework map and all AFLP markers and another 24 SSR markers were inserted. The linkage map consists of 10 linkage groups with a total distance of 1223 cM and an average interval of 5.43 cM.The data of resistant and susceptible phenotypes of 180 F₂ individuals (145 resistant and 35 susceptible) was analyzed with 207 markers in the genetic linkage map of pak-choi by MAPMAKER/EXP 3.0. We inserted TuRBCH01 locus into R6 using'try'command and it was still positioned between EaccMctt 3 (E36M62-3) and EatcMcac 1 (E44M48-1). The interval observed between E36M62-3 and TuRBCH01 is 7.9cM, and 21cM between TuRBCH01 and E44M48-1. Though the intervals between TuRBCH01 locus and linked AFLP markers in the present linkage group are slightly larger, there was no significant difference between them (P>0.05). According to the report of Kim et al. (2009), TuRBCH01 should be on chromosome 5 of pak-choi.In this study, the TuMV-resistant gene of pak-choi was firstly detected and named. A single dominant gene was firstly proved to control TuMV-C5 resistance gene and located on the linkage group R6 in the genetic linkage map of pak-choi. We hope that our research can help to clone TuMV-resistant gene, to study the B. rapa ssp. chinensis genome and to conduct MAS in genetics and breeding.