Bt cotton showed decreased ability to kill cotton bollworm
larvae during flower development and flowering.
Cotton (Gossypium hirsutum).
Inserted Transgenes and Intended Effect:
crylAc gene derived from Bacillus thuringiensis
(Bt). This gene gives the plant the ability to produce an
insecticidal delta-endotoxin that can kill the larvae of insect pests,
including the cotton bollworm (Helicoverpa armigera), which is
a major pest in cotton fields. The crylAc gene was fused to the
cauliflower mosaic virus (CaMV-35S) promoter so that the toxin would be
produced continuously in all parts of the plant.
Goal of This Study:
Based on observations that transgenic cotton plants containing the
Bt gene ("Bt cotton") showed a decrease in efficacy
later in the growing season at the time of peak flowering, the researchers
wanted to investigate what might be causing this problem. They carried
out field and greenhouse experiments in Australia, using commercial
varieties of Bt cotton. They fed the larvae of a laboratory-bred,
Bt-sensitive strain of cotton bollworm on diets containing leaves
of Bt cotton that had been harvested at different times during
the growing season. They also determined the levels of the crylAc
gene expression (mRNA transcript levels) and Bt toxin levels in
the leaves at different times during the growing season. They carried
out similar experiments with field-grown and greenhouse-grown Bt
Results of This Study and Unintended Effects:
The researchers found:
Field-grown Bt cotton leaves showed significantly decreased
ability to kill cotton bollworm larvae in relation to the developmental
stage of the plant. When the plants reached a particular stage of flower
development (after "squaring"), leaf toxicity to larvae decreased
dramatically and stayed low during flowering. This was also the case in
greenhouse-grown Bt cotton.
The lowered toxicity of Bt cotton leaves was clearly correlated
with a decline in the expression of the crylAc gene and reduced
amounts of Bt toxin in the leaves.
Since the Bt gene is supposed to be expressed continually in all
parts of the plant during its complete life cycle, some physiological
or environmental factors must be affecting transgene expression during
Bt cotton development. Decreased ability to kill the cotton
bollworm larvae during flowering increases the likelihood that some
larvae will survive and propagate, which in turn increases the risk
that Bt-resistant strains of the cotton bollworm will evolve:
"Resistance evolves when insects are exposed to a dose that kills
susceptible, but not resistant individuals. If toxin levels fluctuate,
it increases the probability that at some stage insects will be exposed
to a discriminating dose" (p. 1007).
The authors remark: "Thus, any inadvertent lowering of foliar
glycoalkaloids in transgenic potato plants could cause an undesired
increase in susceptibility to those pests which are sensitive to threshold
concentrations of glycoalkaloids for insect deterrence or toxicity,
potentially reducing the benefits of expressing anti-insect transgenes
in plants. . . . We think that it is as important to monitor unintended
changes in the levels of such secondary plant compounds as it is to
evaluate the potential risks and benefits of the intended transgene
product (anti-insect gene products) in the agro-ecosystem" (p. 148).
Olsen, K. M., J. C. Daly, H. E. Holt, and E. J. Finnegan
(2005). Season-Long Variation in Expression of Cry1Ac Gene and
Efficacy of Bacillus thuringiensis Toxin in Transgenic Cotton
Against Helicoverpa armigera (Lepidoptera: Noctuidae),"
Journal of Economic Entomology vol. 93, pp. 1007-17.
Australia's national science agency, CSIRO.
Cotton Research and Development Corporation, Australia; Australian Cotton
Cooperative Research Centre.
Bt cotton is grown around the world and has been on the market
since the late 1990s.
Copyright 2008 The Nature