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Association mapping for novel resistance loci against bacterial spot using genome-wide SNPs in tomato
Bacterial spot of tomato (Solanum lycopersicum L.) is caused by at least four species of Xanthomonas with multiple physiological races. In this study, we developed a mapping population for association analysis of bacterial spot resistance. For this population, six advanced breeding lines with distinct sources of resistance were first crossed in all combinations and their F1 hybrids were intercrossed. The 1,100 segregating progeny from these crosses were evaluated in the field against T1 strains. Based on this individual evaluation, we selected 5% of the most resistant and 5% of the most susceptible progeny for evaluation as plots in two subsequent replicate field trials inoculated with T1 and T3 strains. A total of 461 markers across 12 chromosomes were used for genotyping these selections. Of these markers, an optimized subset of 384 SNPs was derived from the 7,720 SNP Infinium array developed by the Solanaceae Coordinated Agricultural Project (SolCAP). For association analysis to detect known resistance loci and additional novel loci, we used the mixed models with correction for population structure, and found that accounting for kinship appeared to be sufficient. Detection of known loci was not improved by adding a correction for structure using either a Q matrix from model-based clustering or covariate matrix from Principal Component Analysis. Both single-point and haplotype analyses identified strong associations in the region of the genome known to carry Rx-3 (chromosome 5) and Rx-4/Xv3 (chromosome 11). Additional QTL associated with resistance were detected on chromosomes 1, 3, 4, 6 and 7 for T1 resistance and chromosomes 2, 4, and 6 for T3 resistance. Haplotype analysis improved our ability to trace the origin of positive alleles. These results demonstrate that both known and novel associations can be identified using complex breeding populations that have experienced directional selection.
