Prof. Joe Cummins deal with further intricacies on the supposed lack of Bt resistance outbreaks from commercial growing of Bt crops.
My report on the surprising finding that little genetic resistance has been observed in transgenic crops containing transgenes for Bacillus thuringiensis (Bt) toxin to kill insect predators [1] evoked response from Bruce Tabashnik, a leading researcher in the new field of resistance management [2]. I pointed out that there are a number of Bt toxin genes, each of which is highly polymorphic (with many different variants), including some that are synthetic so the absence of resistance breakouts could be related to the diversity and polymorphism of transgenes in the crops, along with the United States sanction of using pesticide sprays in the refuge areas, as both normal susceptible pest and resistant mutants will be killed by the spray.
Tabashnik did not challenge my assertion that the pesticide spray will destroy resistant mutants, but claims that the polymorphism of the transgenic stocks was low, because, for example, most of the millions of acres transgenic corn contain the Bt Cry1Ab gene while transgenic cotton has mainly the Cry1Ac gene. So, the crops should be considered essentially monomorphic, thereby providing a large target for resistance to evolve.
The presumption that transgenes described as Bt Cry1Ab in maize or Bt Cry1Ac in cotton are nearly identical is fallacious. However, the products of the mainly synthetic genes Cry1Ab toxin in maize and Cry1Ac toxin in cotton are similar to the respective product of the natural gene is perhaps justified, but they are not identical. More importantly, the various ways in which the genes specify Cry1Ab or Cry1Ac are synthesized and altered from the natural gene are described in the bio-pesticide registration documents of the Environmental Protection Agency (EPA) [3-6], and the determination of non-regulated status documents of the United States Department of Agriculture (USDA) [ 7-11].
The genes in the transgenic crops produced by different corporations usually differ in the associated regulatory sequences such as promoter, enhancer, terminator and intron. They also differ in codons specifying the amino acid sequence of the final protein, having been altered in different ways by each corporation to enhance production. The documents show that the two main commercial transgenic maize lines differ in production of the protein by about ten fold. Certainly the mutations are influenced by gene activity in two ways, one way is the influence of low level toxin production which allows for greater insect survival. The USDA documents show that there are three separate and distinct gene insertions for Cry1Ab maize and two insertions for Bt Cry1Ac cotton. Thus there is hidden diversity in the transgenic toxin genes. But Cry toxin proteins are presumed to be identical even though their parent genes are dissimilar, which is what Tabashnik has stated [2].
There is yet another caveat. For the most part, toxicity testing of the Cry toxins was not done with the Cry toxin produced in the crops, but with the protein produced in large scale bacterial cultures and purified. This was allowed, even though the genes in the crops had greatly altered DNA sequences. Thus, proteins toxins produced in bacteria were deemed identical to the toxin protein produced in the crop, provided they had a similar molecular size using gel electrophoresis, a relatively crude way of measuring molecular size, that could easily conceal significant differences in the amino acid sequences between the natural and synthetic gene product. This is yet another case of assuming ‘substantial equivalence’ when the available evidence gives no indication that the protein products of the transgenes for the Bt toxins are identical to the original toxin isolated from the bacterium nor, for that matter, to each other.
The structure and evolution of Bt toxins has been studied extensively. Like all proteins, the toxin is separated into domains, the Bt toxin domains are involved in insect membrane insertion and formation of pores (leading to osmotic disruption of the cell) and membrane receptor recognition [12]. Insect resistance is expected to disrupt the interaction between toxin and membrane, but may also involve direct destruction of the toxin. The Bt toxins are highly polymorphic in nature and such variability discourages rapid spread of resistant insects.
In the final analysis, the widespread use of insecticide sprays in the "refuge" set aside to minimize the impact of mutant insects will certainly eliminate most resistant mutants even though such mutants are readily observed in the laboratory experiments. However, the hidden polymorphism of the commercial Bt crops may also reduce the spread of any resistant mutants that survive chemical pesticides. The polymorphism of commercial Bt crops has not received the careful scrutiny by regulators and the academic community that it deserves.
Article first published 16/10/03
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