Exploring factors that may promote the longevity of Rag genes

Despite the genetic bottleneck that Aphis glycines passed through on the way to North America and the sparse use of aphid-resistant soybeans (i.e., Rag genes) in North America, biotypes have been identified that overcome this resistance. These biotypes are defined by their capacity to survive on aphid-resistant cultivars of soybean (e.g., biotype-2 survives on Rag1 soybean). However, there is limited evidence of fitness costs associated with these virulent biotypes. We conducted a series of experiments to determine if there are fitness costs associated with virulent biotypes on susceptible soybean. We evaluated biotype-2, biotype-3, and biotype-4 on susceptible soybean. Our results indicate that fitness costs exist for biotype-2, biotype-3, and biotype-4 on aphid-susceptible soybean. In addition, we also observed negative cross-resistance for biotype-2 on Rag2, and biotype-3 on Rag1 soybean. These results are the first to document fitness costs for biotype-2 and biotype-4 on susceptible soybean, and also the first to document negative cross-resistance of virulent biotypes toward Rag genes that they are not virulent towards.