Tabashnik et al. explained that using transgenic crops such as Bt cotton could significantly reduce the use of chemical insecticides and pesticides. They also pointed out that excessive use of transgenic crops initiated the development of resistance in pests, which must be delayed. They suggested the use of two different Bt toxins to target the same pest, such as integration of Cry1Ac and Cry2Ab in a single crop. Their experiments revealed that cross resistance did occur in some major cotton pests.
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Karanthi et al. analyzed the expression of Cry1Ac in different parts of the plant. They proposed that its expression varied among hybrids as well. Bt cotton leaves exhibited the highest levels of Cry1Ac. The expression of the toxin in the flowers and boll-rind squares was found to be insufficient to protect against pests. Furthermore, they discovered that the expression of Cry1Ac protein was inversely proportional to the age of the plant.
Bakash et al. studied the expression of Cry1Ac and Cry2A genes in transgenic cotton using different techniques of molecular analysis. Quantification of both the insecticidal genes was found to vary among different parts of the plant and among different plant varieties as well. Toxin levels were highest in leaves, followed by petals, anthers, bolls, and finally square. Gene expression was also seen to decline with increasing plant age.
Dhurua and Gujar conducted a survey to analyze the development of resistance to Bt toxins by the pink bollworm. Transgenic cotton expressing either the Cry1Ac or a combination of Cry1Ac and Cry2Ab2 has been found to be very effective in eradicating this destructive pest. Despite this, some other natural enemies have become resistant to these toxins in the fields on India. Results of the study concluded that resistance to Cry1Ac had progressed in a sample of non-Bt cotton obtained from Amreli, Gujarat. The median lethal concentration (LC50) of Cry1Ac required to kill five-day-old insect larvae was 44 times higher than normal.
Tindall et al. elaborated the significance of combining Cry1Ac and Cry1F insecticidal crystal proteins in Bt cotton variants. The potency of this was assessed against different lepidopteran pests, such as fall armyworm,beet armyworm Spodoptera frugiperda,Spodoptera exigua (Hübner), Pseudoplusia includens(Walker) and soybean looper. The study was performed in varying environmental conditions. It was revealed that cultivars containing Cry1Ac:Cry1F had significantly lesser damage in terms of bract feeding, boll abscission, leaf defoliation and penetrated squares and bolls. A much greater mortality rate for fall armyworm in comparison to non-Btcotton plants was also seen (90–100%). The study concluded that cotton cultivars with a combination of both Cry1Ac and Cry1F can be a highly effective mechanism in integrated pest management programs. Furthermore, the issue of differential pest susceptibility can also be tackled using this method.
Zhang et al. proposed that the progression of pest resistance significantly decreases the efficiency of insecticidal Bt toxins. They elaborated that resistance to Cry1Ac due to a recessive mutation in a gene that encodes a toxin-binding cadherin protein has been found through previous studies, but field analysis has not been conducted. Their experimentation was based on detecting recessive cadherin mutations and deletion mutations which gave non-recessive resistance to insects and pests. Recessive cadherin alleles were responsible for 75–84% of all the resistance detected. It was concluded that resistance management strategies must be employed to assess and identify the variety of resistance alleles, as well as non-recessive alleles that have been detected in field experiments conducted in China.
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Iqbal et al. examined the influence of different environmental factors on the expression of Bt gene. They subjected ten Pakistani cotton plants of varying genotyoes to different salt stress conditions. Data regarding the detection and quantification of Bt endo-toxins was recorded using immune strip anaylsis and ELISA, and the presence of Cry1Ac, Cry1F and Cry2Abgenes was also conducted. The results showed that only the Cry1Ac gene was expressed. Varying salt concentrations did effect the expression of the Bt toxin, and exhibited an inverse relation. Moreover, the levels of the toxin were found to decrease 90 days after sowing.
Adamczyk J and Meredith W assessed whether the overall expression of Cry1Ac protein in different commercial varieties of Bollgard cotton was a result of only genetic factors. Such a study could help breeders select those varieties of transgenic cotton which showed superior expression of Bt toxins as well as certain other agronomic characteristics. Two cultivars were crossed under controlled conditions, one expressing the endotoxin at a high level and one showing low expression. The parents, along with the F1 and F2 generations were planted under field conditions, and protein expression was quantified using ELISA.
Jr.Jhon et al. investigated whether environment or genetics played a greater part in the expression of Cry1Ac protein. They considered thirteen commercially available varieties of transgenic Berliner (Bt) cotton, and studied them for endotoxin expression. Results revealed that 2 out of the 11 varieties, namely DP 458B/RR and NuCOTN 33B, Delta & Pineland Co., Scott, MS, showed the greatest expression of Cry1Ac. Both these varieties had the same parental origin. Analysis of the F1 generation showed similar results. Hence it was concluded that genetics had a greater impact on protein expression than the environment.
Torres J, Ruberson J and Adang M performed a study to assess the degree to which Cry1Ac protein expressed in Bt cotton reaches its third trophic level. They also measured the amount of protein that was acquired by herbivores and the amount that was exposed to pests. They conducted experiments under laboratory and field conditions using ELISA. Results showed a decrease in the expression of Cry1Ac as the season progressed. For herbivores, namely lepidopteran larvae, the amount of protein acquired varied among species. For pests and predators, seven species were considered, and Cry1Ac was found in Chrysoperla rufilabris and Podisus maculiventris.
LixU et al. performed a number of different studies with regard to resistance to Bt cotton. They analyzed the time it would take for the development of resistance to Bt toxins with and without the presence of the natural pest. Secondly, they investigated the potency of combining biological control and Bt crops to eliminate pest populations. Thirdly, the possibility that insecticide-sprayed non-Btcrops might repress or accelerate the development of resistance was also assessed. The study comprised of Btbroccoli expressing the Cry1Ac protein, the mildly resistant pest Plutella xylostella, and a natural enemy, Coleomegilla maculate. Experiments were conducted for many generations. Overall analysis of the results revealed that natural pests could indeed delay the development of resistance to Bt plants, and could have a significant use in integrated pest management (IPM) systems.
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