Genotype-specific fitness cost of resistance to Bt toxin Cry1Ac in pink bollworm

BACKGROUND: To improve resistance management strategies for Bacillus thuringiensis (Bt) crops, a better understanding of the relative fitness of pest genotypes with resistance alleles in the absence of Bt toxins is needed. Here, we evaluated the impact of costs of resistance to Bt toxin Cry1Ac on the relative fitness of specific pink bollworm (Pectinophora gossypiella) genotypes. We created two heterogeneous strains with an intermediate frequency of mutant cadherin alleles linked with resistance to Cry1Ac, reared the strains on diet without Bt and tracked the decline in frequency of resistant genotypes for 15–30 generations using polymerase chain reaction amplification. We used a population genetics model and sensitivity analyses to estimate the relative fitness of resistant genotypes. RESULTS: Costs were completely recessive in one strain and almost completely recessive in the other. Estimates of the decline in relative fitness of the resistant homozygotes fed on a diet without Bt were 14–22% in one strain and 21–36% in the other. CONCLUSION: Our genotype-specific cost estimates and the results of studies discussed herein indicate that costs associated with resistance to Bt are often large enough to significantly delay the evolution of resistance to pyramided Bt crops in pests with recessive inheritance of resistance.