HAU16008-08-6 - Contamination of refuges by Bacillus...

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Contamination of refuges by Bacillus thuringiensis toxin genes from transgenic maize Charles F. Chilcutt* and Bruce E. Tabashnik *Department of Entomology, Texas A&M University, 10345 Agnes Street, Corpus Christi, TX 78406; and Department of Entomology, University of Arizona, Tucson, AZ 85721 Communicated by William S. Bowers, University of Arizona, Tucson, AZ, January 23, 2004 (received for review December 11, 2003) Transgenic crops producing insecticidal toxins from Bacillus thu- ringiensis (Bt) are widely used to control pests, but their benefits will be lost if pests evolve resistance. The mandated high-dose/ refuge strategy for delaying pest resistance requires planting refuges of toxin-free crops near Bt crops to promote survival of susceptible pests. We report that pollen-mediated gene flow up to 31 m from Bt maize caused low to moderate Bt toxin levels in kernels of non-Bt maize refuge plants. Immunoassays of non-Bt maize sampled from the field showed that the mean concentration of Bt toxin Cry1Ab in kernels and the percentage of kernels with Cry1Ab decreased with distance from Bt maize. The highest Bt toxin concentration in pooled kernels of non-Bt maize plants was 45% of the mean concentration in kernels from adjacent Bt maize plants. Most previous work on gene flow from transgenic crops has emphasized potential effects of transgene movement on wild relatives of crops, landraces, and organic plantings, whereas im- plications for pest resistance have been largely ignored. Variable Bt toxin production in seeds of refuge plants undermines the high- dose/refuge strategy and could accelerate pest resistance to Bt crops. Thus, guidelines should be revised to reduce gene flow between Bt crops and refuge plants. G enetically modified crops that produce insecticidal proteins from Bacillus thuringiensis (Bt) kill some key pests, but their usefulness will be cut short if pests adapt. Pests have not yet evolved resistance to Bt crops in the field (1). However, many have been selected for resistance in the laboratory, and dia- mondback moth ( Plutella xylostella ) has evolved resistance to Bt sprays in the field (1–3). To counter the threat of resistance, the U. S. Environmental Protection Agency has mandated the ‘‘high-dose/refuge strategy’’ requiring farmers to grow toxin-free crop refuges near Bt crops (ref. 4 and www.epa.gov/pesticides/ biopesticides/pips/bt brad.htm). The purpose of toxin-free ref- uges is to promote survival of susceptible pests. Ideally, rare resistant adults emerging from Bt plants mate with relatively abundant susceptible adults from refuges, and their heterozy- gous progeny are killed by a high dose of toxin from Bt plants. Models predict that resistance will be delayed substantially if these assumptions hold (4), but pollen-mediated gene flow from Bt crop plants to refuge plants could disrupt this strategy.
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