Binding site alteration is responsible for field-isolated resistance to Bacillus thuringiensis Cry2A insecticidal proteins in two Helicoverpa species

Abstract

Background: Evolution of resistance by target pests is the main threat to the long-term efficacy of crops expressing Bacillus thuringiensis (Bt) insecticidal proteins. Cry2 proteins play a pivotal role in current Bt spray formulations and transgenic crops and they complement Cry1A proteins because of their different mode of action. Their presence is critical in the control of those lepidopteran species, such as Helicoverpa spp., which are not highly susceptible to Cry1A proteins. In Australia, a transgenic variety of cotton expressing Cry1Ac and Cry2Ab (Bollgard II) comprises at least 80% of the total cotton area. Prior to the widespread adoption of Bollgard II, the frequency of alleles conferring resistance to Cry2Ab in field populations of Helicoverpa armigera and Helicoverpa punctigera was significantly higher than anticipated. Colonies established from survivors of F(2) screens against Cry2Ab are highly resistant to this toxin, but susceptible to Cry1Ac.

Methodology/principal findings: Bioassays performed with surface-treated artificial diet on neonates of H. armigera and H. punctigera showed that Cry2Ab resistant insects were cross-resistant to Cry2Ae while susceptible to Cry1Ab. Binding analyses with (125)I-labeled Cry2Ab were performed with brush border membrane vesicles from midguts of Cry2Ab susceptible and resistant insects. The results of the binding analyses correlated with bioassay data and demonstrated that resistant insects exhibited greatly reduced binding of Cry2Ab toxin to midgut receptors, whereas no change in (125)I-labeled-Cry1Ac binding was detected. As previously demonstrated for H. armigera, Cry2Ab binding sites in H. punctigera were shown to be shared by Cry2Ae, which explains why an alteration of the shared binding site would lead to cross-resistance between the two Cry2A toxins.

Conclusion/significance: This is the first time that a mechanism of resistance to the Cry2 class of insecticidal proteins has been reported. Because we found the same mechanism of resistance in multiple strains representing several field populations, we conclude that target site alteration is the most likely means that field populations evolve resistance to Cry2 proteins in Helicoverpa spp. Our work also confirms the presence in the insect midgut of specific binding sites for this class of proteins. Characterizing the Cry2 receptors and their mutations that enable resistance could lead to the development of molecular tools to monitor resistance in the field.

Figure 1. Binding of ¹²⁵I-Cry2Ab proteins to BBMV from Helicoverpa spp. revealed by autoradiography.

¹²⁵I-Cry2Ab was incubated with BBMV in the absence or the presence of an excess of competitor, and the pellet obtained after centrifuging the reaction mixture was subjected to SDS-PAGE and exposed to an X-ray film for 10 days. (A) ¹²⁵I-Cry2Ab binding to H. armigera: lane 1, a sample of ¹²⁵I-Cry2Ab protein used in the binding assays; lane 2, ¹²⁵I-Cry2Ab incubated with BBMV in the absence of competitor; lane 3, homologous competition (excess of unlabeled Cry2Ab); lane 4, ¹²⁵I-Cry2Ab incubated with BBMV from SP15-resistant insects. (B) ¹²⁵I-Cry2Ab binding to H. punctigera: lane 1, ¹²⁵I-Cry2Ab incubated with BBMV in the absence of competitor; lane 2, homologous competition; lane 3, ¹²⁵I-Cry2Ab incubated with BBMV from Hp4-13-resistant insects; lane 4, a sample of the ¹²⁵I-Cry2Ab protein used in the binding assays.

Figure 2. Binding of ¹²⁵I-Cry proteins to BBMV from H. armigera.

Binding of iodinated Cry proteins to H. armigera at increasing concentrations of BBMV from GR (•) and ANGR (▪) susceptible strains, and to SP15 (○) and 6-364 (□) resistant strains. Non-specific binding was determined by adding an excess of unlabeled protein to the reaction. Specific binding was calculated by subtracting the non-specific binding from the total binding. (A) Specific binding of ¹²⁵I-Cry2Ab to BBMV from SP15 and its susceptible control (GR) strain. (B) Specific binding of ¹²⁵I-Cry2Ab to BBMV from 6-364 and its susceptible control (ANGR) strain. (C) Specific binding of ¹²⁵I-Cry1Ac. Data points in figures A and B represent the means of two replicates.

Figure 3. Binding of ¹²⁵I-Cry proteins to BBMV from H. punctigera.

Binding of iodinated Cry proteins to H. punctigera at increasing concentrations of BBMV from the susceptible LHP strain (•) and the resistant Hp4-13 strain (□). Non-specific binding was determined by adding an excess of unlabeled protein to the reaction. Specific binding was calculated by subtracting the non-specific binding from the total binding. (A) Specific binding of ¹²⁵I-Cry2Ab. (B) Specific binding of ¹²⁵I-Cry1Ac. Data points in figure A represent the means of two replicates.

Figure 4. Competition binding experiments with H. punctigera BBMV.

Binding of ¹²⁵I-Cry2Ab (A) and ¹²⁵I-Cry1Ac (B) to BBMVs from H. punctigera at increasing concentrations of unlabeled competitor: Cry2Ab (•), Cry2Ae (○), and Cry1Ac (□).