Construction Of A PCR Marker Based High-Density Wheat Map And Development Of Near-Isogenic Lines For FHB Resistance QTLs

Common wheat is a hexaploid species with a large and complex genome. A reference genome marker map (ITMI map) has been constructed with the recombinant inbred line population derived from a synthetic-wheat cross. But it is not sufficient for a full understanding of the wheat genome under artificial selection without comparing it with intervarietal maps. Genetic maps enriched with EST-derived markers are important to apply the genome information to crop improvement.In the present study, using the intervarietal mapping population derived from 'Nanda2419' x 'Wangshibai', we constructed a regional high-density genetic map of wheat with a total map length of 4243.6 cM, comprising 891 loci including 349 detected by EST-derived markers. As a supplement to the ITMI map, this map covered, mostly with the EST-derived markers, about 158 cM chromosomal regions missing in the ITMI map. The missing portion was composed of terminal portions of chromosomes 2BS, 4BL, 5AS and 5BL. In this map two-thirds of the high marker-density regions, including fifteen high-resolution mapping regions, were present in telomeric and interstitial regions. We found that the low marker density regions were largely conserved among cultivars as well as between homoeologous subgenomes. Most of the polymorphic EST-derived markers mapped to distal portions of chromosomes where the marker density and recombination rate is usually higher. QTLs/genes of agronomical importance mainly distributed on group 1, 2, 3 and 5 chromosomes and displayed a clustered distribution toward the telomeric regions. Even though the recombination rate is always low in the centromeric regions, some exceptions existed. The pericentromeric regions on chromosomes 2BL and 3BS had a recombination rate close to recombination hot spots. Alignment of the mapped ESTs with BAC/PAC clones along the rice chromosomes indicated that wheat chromosome groups 3, 6 and 7 had much better collinearity to rice chromosomes than the others. These findings would have important implications in structure and function analysis of wheat genome as well as in wheat gene mapping, cloning, and breeding programs.Type I resistance against initial penetration and type II against fungal spread within spikes are two major types of scab resistance. To map QTLs for these two types of resistance, two independent experiments usually are set up. In this study, we used the number of diseased spikelets (NDS) 14 and 21 days post spraying inoculation to reflect the combined effects of type I and type II resistance. The differences (Dnds) between NDS_D21 and NDS_D14 of each line was used to reflect the type II resistance. We found that NDS_D14 and NDS_D21 data reflected mainly the type I resistance by QTL analysis in the population used. Using D_NDS, we were able to identify all major type II resistance QTLs. Using this mothed, we found the type II QTL on chromosome 2B was also associated with type I resistance.Using a set of 277 recombinant inbred lines developed from the cross between Nanda2419 and Wangshuibai, we re-examined QTLs associated with type I resistance. The major QTLs located on 4B and 5A were mapped to the smaller marker intervals. Their LOD scores also increased greatly compared to the primary results using 136 lines. For example, the LOD score of QFhi.nau-4B increased from 4.8 to 7.6, and from 10.2 to 14.4 for QFhi.nau-5A. These results demonstrated that larger population size is useful for more precise QTL mapping.We also constructed six near-isogenic lines (NJLs) for FHB resistance QTLs through marker-assisted backcrossing. Background selection indicated that the percentage of recipient genome contents (RGC) of NILs was over 97%. The resistance performance of the NILs was significantly different from the recurrent parent Mianyang99-323, and was in accordance with the effect of target QTLs. Through selecting and evaluating recombinants, three major QTLs located on chromosomes 3B, 4B and 5A were mapped precisely. We mapped QFhs.nau-3B to Xbarcl47-Xgwm493 interval, QFhi.nau-4B to Xgwm192-Xgwm149 interval, and QFhlnau-5A to Xgwm415-Xgwm304 interval.