Cotton plants and seeds expressing Bt toxin were found in non-Bt refuges.
Cotton (Gossypium hirsutum).
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.
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.
University of Arizona (Tucson); USDA Agricultural Research Service
USDA Biotechnology Risk Assessment Research Grant.
Bt cotton expressing the Cry1Ac toxin is grown on millions of
acres in the U.S.
Copyright 2009 The Nature