Reduced Fitness of Virulent Aphis glycines (Hemiptera: Aphididae) Biotypes May Influence the Longevity of Resistance Genes in Soybean

Abstract

Sustainable use of insect resistance in crops require insect resistance management plans that may include a refuge to limit the spread of virulence to this resistance. However, without a loss of fitness associated with virulence, a refuge may not prevent virulence from becoming fixed within a population of parthenogenetically reproducing insects like aphids. Aphid-resistance in soybeans (i.e., Rag genes) prevent outbreaks of soybean aphid (Aphis glycines), yet four biotypes defined by their capacity to survive on aphid-resistant soybeans (e.g., biotype-2 survives on Rag1 soybean) are found in North America. Although fitness costs are reported for biotype-3 on aphid susceptible and Rag1 soybean, it is not clear if virulence to aphid resistance in general is associated with a decrease in fitness on aphid susceptible soybeans. In laboratory assays, we measured fitness costs for biotype 2, 3 and 4 on an aphid-susceptible soybean cultivar. In addition, we also observed negative cross-resistance for biotype-2 on Rag3, and biotype-3 on Rag1 soybean. We utilized a simple deterministic, single-locus, four compartment genetic model to account for the impact of these findings on the frequency of virulence alleles. When a refuge of aphid susceptible was included within this model, fitness costs and negative cross-resistance delayed the increase of virulence alleles when virulence was inherited recessively or additively. If virulence were inherited additively, fitness costs decreased the frequency of virulence. Combined, these results suggest that a refuge may prevent virulent A. glycines biotypes from overcoming Rag genes if this aphid-resistance were used commercially in North America.

Strategies for Enhanced Crop Resistance to Insect Pests

“…A key limitation of R gene-mediated resistance in crop protection is that it selects for evolutionary diversification of the insect effector proteins, thereby breaking the resistance. This coevolutionary arms race (Figure 2) has been demonstrated, for example, for Aphis glycines on soybean (139), Nilaparvata lugens on rice (26), and Hessian fly on wheat (125). Pyramiding of multiple R genes can enhance the durability of crop resistance (see, e.g., References 69,120,and 139), although this strategy can depress crop yield.…”

Geographic Distribution and Intrabiotypic Variability of Four Soybean Aphid Biotypes

“…Given that few resistance traits were planted by farmers from 2008 to 2013, it is not clear why the observations were different. Thus, there was little indication of fitness costs due to virulence in the aphid population (Varenhorst et al, 2015). All are widely distributed across the sampled region.…”

“…Biotype 1 represents about a third of total isolates, and about a third of scores 4 to 5 were made by Biotype 1. Thus, there was little indication of fitness costs due to virulence in the aphid population (Varenhorst et al, 2015).…”

Impact of soybean trichomes on Aphis glycines (Hemiptera: Aphididae) and their interaction with natural enemy abundance and predation

“…The two controls used were nearisogenic (Wiarda et al 2012) We used biotype-1 of A. glycines from a colony that was founded from individual females obtained from the Ohio State University. This biotype is avirulent to Rag1 and Rag2 genes, and is unable to persist as colonies on soybean varieties that contain both genes (Varenhorst et al 2015). The A. glycines (biotype-1) colony were maintained within a Percival E41L2C9 growth chamber (Percival Scientific, Incorporated, Perry, IA).…”