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Cotton plants and seeds expressing Bt toxin were found in non-Bt refuges.

Manipulated Organism: Cotton (Gossypium hirsutum).

Inserted Transgenes: Cry1Ac gene derived from Bacillus thuringiensis (Bt). This gene produces a protein that is toxic to the larvae of moths (Lepidoptera). The Cry1Ac gene was fused to the cauliflower mosaic virus promoter (CaMV-35S) to produce the Bt toxin in all parts of the plant.

Goal of This Study: As with any pesticide, the efficacy of the Cry1Ac protein can be undermined over time because Bt cotton plantings select for those rare insect pests with resistance to the toxin. To counteract this tendency, most states require Cry1Ac-producing cotton stands to be co-planted with non-Bt varieties. These non-Bt refuges support susceptible pest populations that mate with resistant pests from the Bt cotton, thereby diluting the frequency of Cry1Ac resistance genes in the overall population. If these refuges are contaminated with cotton plants that express Cry1Ac, however, the effective refuge size is reduced. Based on indications that refuge contamination was occurring, research was conducted in Arizona to quantify its extent and to predict what effects it might have on the evolution of resistance among cotton pests.

Results of This Study:
  • About 8% of the cotton plants in refuges on the university farm and up to 5% of the edge-row plants in commercial farm refuges tested positive for the Bt toxin. Among these adventitious Bt plants, some appeared to contain two copies of the Cry1Ac gene because all of their seeds tested positive for the toxin. Other plants were inferred to contain only one copy of the Bt gene because, on average, 78% of their seeds tested positive for the Bt toxin (a 3:1 ratio is expected).

  • Among the non-Bt plants in the refuges, a small percentage appeared to have outcrossed with Bt plants (from either inside or outside the refuge) because they contained seeds with the Bt toxin. About 10% of the bolls in the university refuges and 15% of the bolls from edge rows in commercial refuges had outcrossed. Outcrossed bolls in non-Bt plants, although not detrimental to foliar pests, will affect cottonseed pests, such as the pink bollworm (Pectinophora gossypiella). Outcrossed bolls are also a source of adventitious Bt plants in future years.

  • According to models of how resistance evolves in pink bollworm populations, the levels of contamination observed in the study are too small to significantly undermine the refuge strategy. The time required for the majority of the bollworm population to become resistant was reduced by less than 10%. This robustness to contamination is dependent on the fact that Cry1Ac resistance is recessively inherited in the pink bollworm (which means two copies of the resistance gene are needed to be effective). For cottonseed pests where one copy of the resistance gene provides some protection against Cry1Ac, such as Helicoverpa zea and H. armigera, the effects of refuge contamination are predicted to be much more detrimental.

Source: Heuberger, S., C. Yafuso, G. Degrandi-Hoffman, B. E. Tabashnik et al. (2008). "Outcrossed Cottonseed and Adventitious Bt Plants in Arizona Refuges," Environmental Biosafety Research vol. 7, pp. 87-96.

Heuberger, S., C. Ellers-Kirk, C. Yafuso, A. J. Gassmann et al. (2008). "Effects of Refuge Contamination by Transgenes on Bt Resistance in Pink Bollworm (Lepidoptera: Gelechiidae)," Journal of Economic Entomology vol. 101, pp.504-14.

Author Affiliations: University of Arizona (Tucson); USDA Agricultural Research Service (Tucson, AZ).

Funding: USDA Biotechnology Risk Assessment Research Grant.

Product Status: Bt cotton expressing the Cry1Ac toxin is grown on millions of acres in the U.S.

Copyright 2009 The Nature Institute.

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