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Resistance to Bacillus thuringiensis Transgenic crops and resistance management strategies
Effective pesticides play a key role in the management of agriculture ecosystems. For example, control of Bollworm in cotton is currently dependent on the use of chemicals. Pesticide usage leads to chemical residues resulting in issues of occupational health and safety. The residue problem is exacerbated by the evolution of resistance to the pesticide. To maintain control, the response is commonly more frequent application of higher concentrations of pesticide with resistance management strategies usually based on general models influenced by the anticipated basis of resistance.
Since the first commercial Bacillus thuringiensis (Bt) formulations appeared in the late 1930's, they have been used widely as niche products for pest control. Bt is a common soil bacterium, most abundantly found in grain dust from silos and other grain storages facilities. It was discovered first in Japan in 1901 by Isahawata and then in 1911 in Germany Berliner. Bt typically produces several types of toxin, for example a-exotoxins (="heat-labile exotoxins"), b-exotoxins (="fly-factor" or "heat-stable exotoxins"), Enterotoxins, vegetative insecticidal proteins. However the d-endotoxins (="Crystal toxins"), are the most important.
The introduction of Bt crops has led to environmental, economic and management changes particularly commercially grown transgenic crops expressing insecticidal Bt Crystal toxins has increased the risk of resistance by providing a constant selection pressure. As many major pests have already developed resistance to chemicals, the fact that Bt makes up to 98% of all biopesticides and that the demand for Bt crops is increasing makes the impact of resistance potentially dramatic.
Management of Bt resistance
Pesticide resistance strategies are related with the interaction between three factors:
(a) Ecological factors like for example, grow rate, predators and parasitic relationship, migration and mobility.
(b) Genetical factors, for example, the initial frequency of resistance genes and pleitropic or dominance relations and
(c) Operational factors in resistance are those related to the application of the pesticides and are thought to be under man's control. Most obviously, these include the timing, dose, and formulation of pesticides used.
Many resistant management strategies have been identified over the past 50 years for synthetic organic insecticides, which focused on different factors. For example, agroecosystem, reduced selection pressure and genetical factors. All of them could be summarized as low dose strategy, multiple attack strategy, high dose strategy and mixture, mosaic or rotation of insecticides.
Few of these approaches have been put into practice. Most of the management were used to improve crop production rather than to manage resistance (e.g. economic thresholds rather than calendar spray schedules). In general, resistance was most effectively delayed by reduction in pesticide use and rotation of pesticides, but not by use of pesticide mixtures.
Probably the reason of the relatively low use of resistant management strategy was of the complexity of management, it is an evolutionary process in the field, plus economic and social constrain (For example pesticide manufacturers, farmers, regulators etc).
Resistant management to Bt transgenic crops has much in common with insect resistance management to chemical, virus, or nematodes insecticides through various strategies, all of which assume that the frequency of resistance alleles will decrease when the selection pressure is reduced or discontinued. The major strategies include (a) mixtures, mosaics or rotations of transgenic plants; (b) time- or tissue-specific expression of toxin; (c) low doses of toxin in combination with natural enemies; (d) co-expression of different cry genes; and (e) high expression (dose) with refugia, which is the strategy recommended currently.
It was a popular idea that a low expression level could be good for managing resistance because it would allow some susceptible individuals to survive but simulation models have shown that one must allow a large proportion (>20 percent) of the treated individuals to survive if resistance is to be significantly delayed. In general, this is impractical because it will allow high damaged crops. Further, the Bt transgenic crops are generally aimed to kill 100% of the targeted insects that feed on these crops. Unfortunately, some of the targeted pests especially cotton bollworms, Helicoverpa armigra are relatively less sensitive to Bt toxins. For example, in Australia about 5% of Helicoverpa armigra survive on the Bt cotton.
High expression (dose) with refugia resistant management is used widely and it was the first comprehensive management plan to be developed for an insect control product before market production. With a high dose toxin exposure, as occurs with transgenic plant, it is difficult for an insect to avoid exposure by cessation of feeding for short time. On contrary, both foliar and systemic insecticides gradually decay as they are exposed to sun light, high temperatures, and irrigation water. Moreover, High Dose Refugia Strategy does not depend on reaching an economical threshold of allowable insect damage to apply the insecticide. For example, Rosenheim and Tabashnik (1990) found that, the time to resistance decreases as the number of generation per unit of time increased when economic threshold is consider.
1- The high dose - refugia strategy is based on three assumptions:
a) That resistance will be inherited as a recessive trait. That is, the majority of the heterozygous progeny will be disabled or killed at the same dose as for homozygous susceptible larvae, thereby limiting the spread of the resistance alleles in the population.
b) That insects with resistance to Bt will be rare initially and will almost always mate (at random) with susceptible wild-type insects giving rise to heterozygous progeny.
c) That resistant individuals will be at a competitive disadvantage due to a fitness cost incurred by carrying the resistance allele.
The refuge concept is designed to ensure that mating of any resistant insects takes place with susceptible beetles. In this context, refugia are areas consisting of non-transgenic plants that will support sufficient homozygous susceptible insects to mate with the majority of homozygous resistant individuals, so that most progeny are heterozygous susceptible. The spatial and temporal location of the refugia in relation to the Bt crop has not been studied in any detail and current regulations do not require the refuge to have any specific configuration. The refugia involve two factors:
1. The location of the refugia should facilite the movements of individuals closer is better - within or adjacent to Bt field.
2. Timing: mating should take place at the same time to ensure mixed matings.
Recent studies have provided evidence to question to some degree the High Dose Refugia assumptions. For example, some studies had shown resistance to Bt was incompletely dominant (Tabashnik et al., 1997; Huang et al., 1999; Sayyed et al., 2000, 2001). While developmental asynchrony in Bt-resistant populations of P. xylostella and the Pink bollworm, Pectinophora gossypella on Cry1Ac treated leaves and Bt transgenic plants compared with non-Bt crops was shown respectively (Darby, 1998; Liu et al. 1999; Sayyed and Wright, 2001). This developmental asynchrony could lead to non-random mating. There are also limited data to suggest that most P. gossypella males disperse 400 m or less from release sites in transgenic cotton crops (Tabashnik et al., 1999) which was not sufficient to distribute wild males randomly between Bt and non Bt-cotton fields. Finally, the assumption that resistant insects are at a competitive disadvantage has also been questioned. For example, the larvae of P. xylostella that evolved resistance to foliar spray of Bt subsp. kurstaki and Cry1Ac had shown no apparent fitness costs where developmental time, fecundity and viability were evaluated (Tang et al. 1999; Sayyed and Wright 2001).
In spite of the above questioning field observation and laboratory bioassay conduced since 1996 have not consistently show any field tolerance or resistance developmental by tabocco budworm, Heliotis vicerans, cotton bollworm, Helicoverpa zea, and pink bollworm Pectinophera gossypella anywhere in Bt transgenic cotton fields in USA (Tabashnik, et al. 2000; Moar, 2002).
2- Interactions of high expression (dose) with refugia strategia with others population genetics, ecological and operational parameters involved in the evolution of pest resistance.
In the Bt crop we have a controlled and constant dose expression of the insecticide which made the evolution of allele resistance management easier compare with other methods of pesticide control. However, a constant selection pressure made not predictable the frequency of resistance allele by itself. We need know the population genetics, ecological and operational factors landscape in which the insect evolve. For example, the assumption that the frequency of resistance will decrease when the selection pressure is removed is generally true. However, as Cerda et al, 2002 shown, the rate of change can vary depending upon the ecological conditions and the population initial level of resistance. Other parameters plus the selection pressure are requires to made the High Dose-Refugia management of evolution of resistance allele predictable. Next we mention which we consider more important for the High Dose Refugia compare with resistance to non engineering plant
The insect movement determine to a large degree the population structure, especially as regards effective population size and genetic differentiation. However the relationship between adult movement and gene flow is a complex one, with no simple one-to-one relationship. Per example eggs laying pattern determine where immature pest are located and can have a strong influence on the overall distribution of insect within a field or region. Insect are mobile and if resistant insects are not fit and this is expressed as less movement, then this could influence the spatial pattern of the refugia (Gould and Tabashnik, 1998).
However, the spatial and temporal location of the refugia in relation to the Bt crop has not been studied in any detail and current regulations do not require the refuge to have any specific configuration. Per example in Bt corn, the EPA requirements are that refuges should be planted within half a mile of fields of Bt corn (Hargrove 1999) without more specifications The effect of the spatial pattern of the refugia on the rate of increase of resistance should be closely related to the movements of susceptible insects from neighboring subpopulations inside the field. Different spatial configurations should affect the spread of R alleles within the population
There are many predators, parasites, and pathogens (viruses, fungus, bacteria, protozoa) associated with the arthropod pest of agriculture crops and abiotic factors, as temperature and precipitation, that reduce pest resistant population growth. All mortality factors are important in Resistance Management because the fitness cost of be resistant could disproportionately reduce the chances of rare resistant individuals surviving to pass genes to the next generations.
Sublethal exposure to transgenic insecticidal crops expressing Bt proteins has been found to slow insect developmental and prolong the duration of immature stages (Liu et al., 1999). A delayed developmental, reduce pest population grow rates and intensifying the activity of mortality factors, particularly abiotic factors as weather enhancing the fitness cost. Moreover if insects within the Bt crop develop to adult stages more slowly than insects outside the crop, mating may be asynchronous and the effective of a refuge will be greatly reduced.
FUTURE APPROACHES:
The industry is working to develop varieties that pyramid two dissimilar toxin genes in the same plant Transgenic insecticidal crop expressing two or more insecticidal proteins that act on independent target sites increase the selection pressure against the pest (Fitt, 2000). Commercialized Bt corn actually express toxin cry1Ab, cry1Ac or cry9c but individually. The influence of more than one toxin is particularly significant if the toxins are not closely related and if the resistance to one toxin does not condition resistance to the other (i.e., cross-resistance). Multiple toxin transgenic cultivars would likely result in high pest mortality and low survival of resistance genes.
Theoretical model suggest that this method have the potential to delay resistance much more effectively than single toxin plants or in mosaic or seed mixtures (Gould, 1991; Roush 1997, 1998) and recent lab and greenhouse experimental result with Pectinophera gossipella show that resistant pink bollworm had little or no survival in cotton with Cry1Ac plus Cry2Ab (Tabashnik et al. 2002) . However, as with High Dose Refugia approach with a single toxin, recessivity is critical in these models, because the added effectiveness of pyramiding is due largely to the raririty of a double resistant homozygote genotype in the population.
Another approaches for future resistance management are a) inducible instead of constitutive expression of Bt alleles. Induction triggered by application of an chemical will provide the flexibility of expression of Bt protein only when it is needed b) selection of plant genotypes with high capacity of compensation with a low expression doses (Fitt, 2000) and c) Conditional expression directed by a wound-inducible promoter (Breitler et al, 2002), which could allow its production both only where and when it is needed, i.e. at the attack site
CONCLUSION:
The originality of the High Dose Refugia relay completely in the control of selection pressure and it is an strong strategy for resistance management compare with resistance management of non-engineered plant. It is because, when selection pressure is extraordinary high natural selection operating on a favoured allele at a single locus is one of the most predictable model of evolution that theoretically exits. However, as show above in this review, ecological and operational factors both in time and space must be considered.
First, the rate at which a resistance allele increase in frequency in a population depends greatly on its degree of dominance. Second that the refugia produce enough susceptible individuals to greatly reduce the probability that the rare resistant individuals mate each other depend of the spatial-temporal present of the refugia and of resistant pest biological parameters as the eggs laying distribution and insect movement. Third the fitness cost and the stability of resistance depend of the biotic and abiotic conditions doing its effect in the resistance evolution difficult to predict.
The above highlighted the conclusion that High Dose refugia strategy for engineered plant it is more reliable and easily to apply compare with spraying chemical insecticides to non engineered plant and in real situation of ongoing insect evolution to insecticide resistance, with enough agro-ecological knowledge the High Dose Refugia management program could be modified to accommodate the effect of other parameters like eggs laying or degree of dominance.
Source: Business Recorder
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