The Associated Effects of the Soybean Aphid Resistance Gene Rag2 from PI 200538 on Agronomic Traits in Soybean

The soybean aphid (SA) (Aphis glycines Matsumura) has become a major insect pest of soybean [Glycine max (L.) Merr.] in North America. Three SA biotypes and several genes conferring resistance to this pest have been discovered. The major SA resistance gene Rag2 was identified and mapped from the maturity group VIII PI 200538 and the gene has been introgressed into Midwest-adapted germplasm. The objective of this study was to test whether Rag2 has unintended effects on seed yield and other agronomic traits in elite midwestern U. S. adapted soybean backgrounds. Homozygous resistant (Rag2) and susceptible (rag2) backcross lines from two backgrounds were tested in multiple environments under conditions with nondetectable SA infestations. Across environments, the Rag2 locus had no significant (P = 0.05) associated effects on seed yield, maturity, plant height, and lodging score in the first background. In the second background, a significant effect was observed across environments for yield, with those lines homozygous for the Rag2 resistance allele averaging 256 kg ha(-1) less yield than lines homozygous for the susceptible allele. The inconsistency of effects observed in the two populations may be the result of differences in the size of the introgressed region in the two populations or the result of background effects. These results show the importance of considering the associated effects of Rag2 when this gene is bred into elite genetic backgrounds.