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Transgenic Bt Technology: 2. Bt Crop Varieties
Prof. C. Kameswara Rao
Foundation for Biotechnology Awareness and Education,
Bangalore, India
krao@vsnl.com, www.fbae.org, www.fbaeblog.org

Specific Bt toxin encoding genes were isolated from Bacillus thuringiensis and incorporated into the genetic complements of several crop plants such as cotton, corn, rice, tomato, potato, soybean, and others, to develop transgenic Bt varieties, using complex yet elegant procedures of genetic engineering.     This results in a crop variety with a single systemic insecticide that kills specific caterpillars feeding on the respective crop.   For each crop the most damaging pest has been targeted, as for example, the bollworms of cotton, and the stem borers of rice and corn.   The objective is that, while the Bt toxins take care of the major pests, the rest can be controlled by conventional practices.  

The choice of Bt genes depends upon the crop and the targeted pest, as most of the Bt toxins are insect group specific.   For example, the proteins encoded by the genes Cry1Ac and Cry2Ab control the cotton bollworms, Cry1Ab controls corn borer, Cry3Ab controls Colarado potato beetle and Cry3Bb controls corn rootworm.    

Transgenic Bt varieties:  A gene construct (or a cassette) consisting of the chosen Bt gene is made, along with other molecular components needed for its expression in the transgenic crop variety.   The construct consists of sequences of nucleotides (the building blocks of DNA, the genetic material) to initiate the expression of the selected gene, to promote such expression, the actual sequence for the gene and a nucleotide sequence to signal the termination of the process of expression.   This construct is then incorporated into the tissue of a (chosen primary) variety of the crop, and this is called an event.   A large number of plants are developed from the event, through micropropagation (tissue culture) for agronomic and regulatory testing processes.   Since this primary variety may not be suitable for cultivation in all countries or even in different regions in the same country, the event has to be transferred into the genetic component of other varieties suitable for cultivation in different parts of the world.   For example, the event MON 531, containing the Cry1Ac gene, was used to develop the Bt cotton variety Coker 312, which is not suitable for cultivation in India.   The chosen Indian regional varieties were repeatedly backcrossed with Coker 312 to develop different Bt cotton varieties.   All Bt cotton varieties containing Cry1Ac gene and developed from MON 531 are marketed under the trade name Bollgard.   In India now 20 Bt cotton varieties are permitted for commercial cultivation in different parts of the country and all of them are Bollgard varieties as they were developed from MON 531 and contain Cry1Ac gene, under license from Monsanto and its partner Maharashtra Hybrid Seed Company (Mahyco).

Acquired resistance and refugium: A prolonged exposure to a toxin at sub-lethal doses may result in the development of gene-based resistance in organisms.   This is called acquired resistance.   Famous examples of such acquired resistance are mosquitoes resistant to DDT and human pathogenic bacteria resistant to antibiotics, which are so casually used, particularly in the developing countries.   There is a distinct possibility of crop pests acquiring genetic resistance to Bt toxins in Bt crop varieties, due to natural variation in susceptibility to a particular toxin, in the caterpillar populations.   Nevertheless, in the past decade of cultivation of Bt cotton, there is not even a single instance of acquired resistance of bollworm to Bt toxin.

In order to de-accelerate the development of acquired resistance, the regulatory frame work in all countries has stipulated that a certain number of rows of the isogenic non-Bt plants should raised along with the Bt crop and this is called the refugium (border or barrier).   A certain number of the caterpillars feeding on Bt plants may escape death and if there was mating among these worms, the resulting progeny would be resistant to Bt toxins to various degrees.   Acquired resistance is a very slow process but may build up to alarming levels if such mating continues for several generations.   The caterpillars feeding on the non-Bt refugium are not exposed to the Bt toxin and so would be susceptible to it.   In the presence of a refugium, a certain proportion of the progeny would be from the mating of Bt-exposed and Bt-unexposed worms, and this progeny would be less resistant to the Bt toxin than the progeny from Bt-exposed worms.    The refugium is thus expected to retard the pace of acquired resistance.  

Cotton farmers are reluctant to lose the product form the non-Bt refugium and often no refugium is planted.   Cotton bollworms also feed on several other crops (polyphagous) and do not seriously affect the commercial product in them.   A non-cotton refugium in a cotton field will function as well as a cotton refugium and should be a viable alternative.      

Gene stacking: Most transgenics contain only one gene, such as for pest tolerance or herbicide tolerance.   In order to compound the benefits, more than one gene is used in the development of a transgenic, by gene stacking or pyramiding.   Transgenic cotton containing both Cry1Ac and Cry2Ab is being developed.    Possibilities are being explored to incorporate both pest and herbicide tolerance in the same variety.   In future, there would be transgenic varieties with three or even four different genes stacked.

Gene stacking can also occur in nature.   If two transgenic varieties of the same crop are resistant to a different herbicide each, intercrossing of these two varieties may result in a hybrid resistant to both the herbicides.