Genetic variability of Spodoptera frugiperda Smith (Lepidoptera: Noctuidae) populations from Latin America is associated with variations in susceptibility to Bacillus thuringiensis cry toxins

Abstract

Bacillus thuringiensis strains isolated from Latin American soil samples that showed toxicity against three Spodoptera frugiperda populations from different geographical areas (Mexico, Colombia, and Brazil) were characterized on the basis of their insecticidal activity, crystal morphology, sodium dodecyl sulfate-polyacrylamide gel electrophoresis of parasporal crystals, plasmid profiles, and cry gene content. We found that the different S. frugiperda populations display different susceptibilities to the selected B. thuringiensis strains and also to pure preparations of Cry1B, Cry1C, and Cry1D toxins. Binding assays performed with pure toxin demonstrated that the differences in the toxin binding capacities of these insect populations correlated with the observed differences in susceptibility to the three Cry toxins analyzed. Finally, the genetic variability of the three insect populations was analyzed by random amplification of polymorphic DNA-PCR, which showed significant genetic diversity among the three S. frugiperda populations analyzed. The data presented here show that the genetic variability of S. frugiperda populations should be carefully considered in the development of insect pest control strategies, including the deployment of genetically modified maize in different geographical regions.

FIG. 1.

Characterization of the selected Bacillus thuringiensis strains. (A) SDS-PAGE of spore-crystal suspensions of selected Bacillus thuringiensis strains. (B) Agarose gel electrophoresis of the plasmid profile present in selected Bacillus thuringiensis strains.

FIG. 2.

Homologous competition binding assays on BBMV isolated from Manduca sexta larvae. Biotinylated trypsin-activated Cry toxins were incubated with the BBMV in the absence or in the presence of a 500-fold excess of unlabeled toxin. After 1 h of incubation, unbound toxins were removed, and vesicles containing bound toxins were loaded onto an SDS-PAGE gel and blotted onto a nitrocellulose membrane. Labeled proteins were visualized by means of a streptavidin-peroxidase conjugate.

FIG. 3.

Genetic variability among the three S. frugiperda populations. Shown is a dendrogram obtained from RAPD-PCR analysis of 10 fourth-instar larvae from each S. frugiperda population.