The soybean aphid (Aphis glycines Matsumura) has become a major pest of soybean [Glycine max (L.) Merr.] in North America since it was first discovered in 2000. The best approach to effectively control evolving biotypes of soybean aphids is to stack resistant genes from different sources using marker-assisted strategies. Plant introduction (PI) 567598B possesses strong antibiosis resistance to soybean aphids. Previous work revealed that two recessive genes control the aphid resistance in PI 567598B. The first objective of the study was to identify the location of the two recessive genes in the soybean genome by genetic linkage analysis of a mapping population consisting of 282 F4:5 lines evaluated in the field and greenhouse. The QTLs controlling soybean aphid resistance were mapped on chromosomes 7 and 16 and named rag1b and rag3, respectively. The two QTLs explained over 30% of the total phenotypic variation. In all trials, rag3 consistently conferred resistance while a soybean aphid isolate overcame rag1b in the field.The next objective was to fine map the aphid resistance genes by selecting plants with recombination events within the QTL intervals. Fine mapping was conducted by screening 4,041 BC1F2-derived lines using SNP markers that flank the interval of rag3. Fifty-five recombinants were tested with high-density molecular markers using the SoySNP50K genotyping array. Progenies of each recombinant were rated for damage by the soybean aphid and tested using custom-designed SNP assays. Results of fine mapping delimited rag3 into a 152-kb interval between SNP markers on chromosome 16. The study showed that effectiveness of rag1b is dependent on the presence of alleles for rag3. Eight candidate genes found within the rag3 interval are encoded as NBS-LRR genes; their potential role in soybean aphid resistance in PI 5657598B will be discussed.
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