Effects of Pathogenic Roundworms on Evolution of Pink Bollworm Resistance to Cry1Ac

Evolution of resistance to Bt toxins is a hindrance in the efficacy of Bt crops. However, fitness cost may cause a delay in the evolution of resistance. Aaron Gassman from the University of Arizona and team conducted a meta-analysis of their four studies and found out that a pathogenic roundworm (Steinernema riobrave) caused a 20 percent fitness cost for pink bollworm larvae that were homozygous for resistance to Cry1Ac, but no significant fitness cost was found for heterozygotes.

The team conducted another set of greenhouse and laboratory selection experiments to find out if the roundworm has the ability to delay the evolution of pink bollworm resistance to Cry1Ac. They imitated the high dose/refuge setting in the greenhouse with Bt cotton plants and refuges (non-Bt cotton plants), and in the laboratory with diet containing Cry1Ac and refuges of untreated diet. For both settings, half of the replicates were exposed to the roundworm, while the other half were not.

Results showed that the roundworm did not delay resistance in the greenhouse. Resistance was delayed in the laboratory setup after two generations but not after four generations. The team also conducted simulation modeling which showed that an initial resistance allele frequency and population bottlenecks can remove the resistance-delaying effects of fitness costs. Thus, the team hypothesized that such factors may have reduced the resistance-delaying effects of the pathogenic roundworm in the selection experiments. The results of the experiment and modeling imply that pathogenic roundworms could delay the evolution of pest resistance to Bt crops, but only under some conditions.

The article is available for Journal of Economic Entomology subscribers at http://www.bioone.org/doi/abs/10.1603/EC11376.