Fine Mapping Of The Major QTL-BraDM For Resistance To Downy Mildew At Seedling Stage Of Chinese Cabbage

Chinese cabbage(Brassica rapa ssp.pekinensis)has increasingly become a worldwide vegetable crop in recent years.According to the statistics of ministry of agriculture,Chinese cabbage production area is about 2.67 million hectares,account for about 15%of all plantings of the vegetables in China.It plays an important role in vegetable supply and increasing farmers' income in our country.However,downy mildew is a serious disease in the main producing areas of China,and the incidence area growing.Therefore,breeding varieties with resistance to downy mildew has become one of the main objectives in Chinese cabbage breeding.Genetic map has a very important role in studying genome structure and localizing genes of interest in plants.Identification and pyramiding of different sources of resistance QTL and breeding resistance materials by molecular design and molecular assisted selection would be the effective ways for breeding stable and lasting disease-resistant varieties,and to ensure the safety of Chinese cabbage production.Our research constructs a high-density bin map with a highly susceptible line 91-112,a line T12-19(highly resistant to downy mildew)and the DH population derived from microspore culture of F1(91-112×T12-19)for Chinese cabbage downy mildew seeding resistance QTL analysis.The main results were summarized as follows:1.3482 of the polymorphic markers and 206 previously-published SSR and InDel markers were used in genetic map construction.The final high-density bin map included 1064 bins on ten linkage groups and was 858.98 cM in length.Through the bin map,we identified six QTLs that were involved in downy mildew resistance,at the seedling,young plant,rosette,and heading stages,and four of these were located in A08 groups.Based on a genetic map derived from a doubled-haploid(DH)population,a major QTL,BraDM,controlling seedling resistance,was identified and localized to the A08 linkage group of B.rapa(Yu et al.,2009),but the confidence interval is still large.For marker assistant selection and molecular cloning the resistance gene,fine mapping of the QTL is very necessary.Here,two DH lines,DH-60(susceptible to downy mildew)and DH-88(resistant to downy mildew)with the similary genetic background but heterozygous at BraDM loci.To fine mapping BraDM,this two DH lines was crossed,and DH-60 was used as recurrent parent,and screening resistant plants to carry out isolated microspore culture,all of this is to construct a DH population including 232 lines.The main results were summarized as follows:2.According to the genome resequencing of the two parents 91-112 and T12-19,11 primer pairs showed polymorphism between parents were developed,and then were analyzed in the DH232 population,we found that BraDM was limited to a interval of 2.6 cM,between the markers Bra17865446 and bru1209,the physical distance between Bra17865446 and bru1209 is 340.085 kb,and there were 11 candidate genes associated with resistance to downy mildew.In order to further study of the related to Chinese cabbage downy mildew seedling resistance loci,a total of 202 Chinese cabbage and 960 specific locus amplified fragment sequencing(SLAF)markers were selected for genome-wide association studies(GWAS).The main conclusions are as follows:3.A single SLAF locus,SLAFMarkerA0124655323,on chromosome A01 were significantly associated with downy mildew resistance.Based on the two nearest ALAF markers that flanking both sides of SLAFMarkerA0124655323,the candidate physical interval was limited to A01:24573724-24755150.4.A single nucleotide polymorphism(SNP)was converted into KASP system applicable marker,and the accuracy of selection in the natural population was 89.9%.