Abstract
Fitness costs of resistance to Bacillus thuringiensis (Bt) crops occur in the absence of Bt toxins, when individuals with resistance alleles are less fit than individuals without resistance alleles. As costs of Bt resistance are common, refuges of non-Bt host plants can delay resistance not only by providing susceptible individuals to mate with resistant individuals, but also by selecting against resistance. Because costs typically vary across host plants, refuges with host plants that magnify costs or make them less recessive could enhance resistance management. Limited understanding of the physiological mechanisms causing fitness costs, however, hampers attempts to increase costs. In several major cotton pests including pink bollworm (Pectinophora gossypiella), resistance to Cry1Ac cotton is associated with mutations altering cadherin proteins that bind this toxin in susceptible larvae. Here we report that the concentration of gossypol, a cotton defensive chemical, was higher in pink bollworm larvae with cadherin resistance alleles than in larvae lacking such alleles. Adding gossypol to the larval diet decreased larval weight and survival, and increased the fitness cost affecting larval growth, but not survival. Across cadherin genotypes, the cost affecting larval growth increased as the gossypol concentration of larvae increased. These results suggest that increased accumulation of plant defensive chemicals may contribute to fitness costs associated with resistance to Bt toxins.
Highlights
Corn and cotton engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt) can increase agricultural profitability while reducing reliance on insecticide sprays [1,2]
Analysis of global monitoring data suggests that host plants that do not make Bt toxins and grow near Bt crops can reduce the risk of resistance [2,3,4]
We focused here on the effects of the cotton defensive compound gossypol on fitness costs by feeding Bt-resistant and susceptible pink bollworm larvae on diet without Bt toxin that either contained or lacked gossypol
Summary
Corn and cotton engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt) can increase agricultural profitability while reducing reliance on insecticide sprays [1,2]. Analysis of global monitoring data suggests that host plants that do not make Bt toxins and grow near Bt crops can reduce the risk of resistance [2,3,4]. Such non-Bt plant ‘‘refuges’’ can provide many susceptible individuals to mate with the rare resistant individuals surviving on Bt crops, yielding hybrid offspring. Refuges are expected to delay resistance most effectively when resistance is inherited as a recessive trait, so the hybrid offspring are killed on Bt crops [4,8,9]. The mechanisms underlying variation in costs among host plants are not well understood, which hampers attempts to identify or create refuge plants that magnify costs
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