Yield drag associated with the soybean aphid resistance gene Rag2 from PI 200538

The soybean aphid (Aphis glycines Matsumura) is an exotic pest of soybean [Glycine max (L.) Merr.] that was first identified in North America during 2000. The pest can be controlled by the resistance genes Rag1 and Rag2, which have been introgressed into Midwestern-adapted soybean lines. In previous studies, the Rag2 resistance allele was shown to be associated with a seed yield reduction. The objective of our study was to confirm the reduction associated with Rag2 and determine its cause. This was done by testing a population of lines with all combinations of resistance and susceptibility alleles at Rag1 and Rag2 to further discern the effect of the Rag2 gene on agronomic traits, and by evaluating the introgressed genetic region surrounding Rag2 in populations of near-isogenic lines to localize quantitative trait loci (QTLs) causing yield drag associated with the Rag2 gene. The populations were tested in multiple environments across 3 yr with little or no aphid infestation. Yield tests of the population segregating for Rag1 and Rag2 showed that Rag2 was associated with a yield reduction across environments, but Rag1 was not. Results from the tests of the Rag2 genetic region populations indicate that one or more QTL(s) causing yield drag are located within a 6.5-Mbp region ~1.6 Mbp from Rag2. This information can be used to break the linkage between the yieldreducing QTL(s) and Rag2, ultimately providing higher-yielding, aphid-resistant cultivars.