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Title: A PCR screen of field populations of Heliothis virescens for a retrotransposon insertion conferring resistance to Bacillus thuringiensis toxin

item Lopez, Juan De Dios

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2006
Publication Date: 2/1/2007
Citation: Gahan, L.J., Gould, F., Lopez, J., Micinski, S., Heckel, D. 2007. A PCR screen of field populations of Heliothis virescens for a retrotransposon insertion conferring resistance to Bacillus thuringiensis toxin. Journal of Economic Entomology. 100:187-194.

Interpretive Summary: Genes from the bacterium Bacillus thuringiensis (Bt) have been inserted into cotton (“Bt cotton”) to produce proteins toxic to several pest insects including tobacco budworm. Bt cotton is widely planted in the US and has significantly reduced insect damage from chewing caterpillars. However, tobacco budworm strains resistant to Bt toxins have been developed in the laboratory and the resistance-causing genetic mutation is known for one strain. To determine whether this mutation occurs in field populations of the tobacco budworm, DNA testing was performed on more than 7000 male moths captured in pheromone traps. The mutation associated with Bt resistance was not found in the field samples, suggesting that the Bt resistance management plan mandated by the US Environmental Protection Agency has been successful. However, because other strains are theoretically possible, continued efforts are required to detect them as well if the DNA-based methods are to be useful.

Technical Abstract: The evolution of pest resistance to transgenic crop plants producing insecticidal toxins from Bacillus thuringiensis Berliner (Bt) poses a continuing threat to their sustainable use in agriculture. One component of the USA-wide resistance management plan for Bt cotton involves monitoring the frequency of resistance alleles in field populations. However, existing methods are expensive and may not detect recessive resistance alleles until their frequencies are too high for countermeasures to be effective; therefore more sensitive methods are needed. The first Bt-resistance-causing mutation described at the molecular level was a retrotransposon insertion into the gene encoding a 12-cadherin-domain protein expressed in the midgut of larval Heliothis virescens F. We report the first large-scale screen for this mutation using a PCR-based approach on more than 7,000 field-collected individuals. The specific insertion was not detected in any of these samples; nor was it detected in three progeny-tested field-caught males thought to carry a Bt resistance gene. Unlike the targets of many chemical insecticides where a limited number of resistance-causing mutations compatible with viability can occur; a very large number of such mutations appears possible for the 12-cadherin-domain gene. However, even if these mutations are viable in the laboratory, they may not threaten the effectiveness of transgenic crops because of a high fitness cost in the field. The challenge remains to detect the subset of possible resistance-conferring alleles that are still rare, but are viable in the field and increasing due to selection by Bt-cotton. This will complicate PCR-based Bt-resistance monitoring strategies.