Risk assessment for insect resistance transgenes

Insect resistance (IR) transgenes offer the advantage to agricultural plants of protection from herbivory. There concern that should IR transgenes escape from the agricultural setting through pollen or seed flow, the advantage conferred by the transgene will not only allow them to persist, but to 'take over' natural populations. Transgene take-over is seen as a problem in that a transformed plant may be unavailable to the herbivores natural to the system, making them more vulnerable to extinction through lowered population size, and so on up the food chain. To address the concern that resistance transgenes might persist in the natural system, Kelly et al. have produced an analytical model targeting the ecological interaction between IR transformed and untransformed plants in the natural community. Successful establishment of a novel allele in a population is a combination of not just the allele's ability to disperse through pollen and propagule, but also the novel allele's compatibility with the genome of the natural population and the ecological dynamic between plants with and without the new allele, i.e., effective gene flow. Kelly et al.'s analytical model specifies the overall character of interactions between factors, allowing predictions outside the range of tested conditions. By so doing, the model not only assesses the risk of any particular transgene, it also identifies points in the dynamic that are sensitive to or may best reward manipulation for control of transgene impact. Kelly et al. approached the problem of insect resistance transgenes in natural populations by recognizing that temporal fluctuations are the central character of the ecological dynamic: year to year variability in herbivory is the rule in both natural and agricultural systems. The appropriate class of models is therefore a storage dynamic, called so because the long-term persistence of a population through periods of low reproduction is 'stored' in either long-term reproductive capacity or dormant propagules. Storage models focus on the probability of recruitment into the reproductive class. The action of the selective factor is thus most important at immature stages of the plant, a schedule also consistent with herbivory, where the same amount of herbivore damage can kill young or small plants but has little effect on larger, mature plants. For a competitive interaction between plants with and without an IR allele, the dynamic is a two-member lottery model comprising one plant type that is more sensitive to the selective factor than the other—the untransformed and transformed lineages, respectively. Differential sensitivity dictates that when the selective factor is present (here, herbivory), the resistant type has the advantage. However, when the selective factor is absent, the resistant type has no advantage, and if there is any net cost associated with resistance, that cost will set the resistant type at a competitive disadvantage for the period the selective factor is not acting. Whether transformed and untransformed plants may stably coexist and in what relative abundances or whether the IR transgene will take over the population depends on 1) the relative frequency of good and bad conditions (high and low herbivory), 2) the relative advantage the IR transgene gives a transformed plant, and 3) the relative disadvantage, if any, the IR transgene carries with it. The differential sensitivity (DS) model was applied to oilseed rape (OSR), where it was found that under levels of herbivore variability established in the field, it takes relatively little disadvantage of carrying the transgene to limit domination of the natural population by the IR allele. This may be treated as a general conclusion. However, the larger point is not that IR transgenes may be relatively easily contained. Rather, the ecological model provides a tool with which to determine how best to do this, as well as to assess how well it has been done. In the model, all terms are ratios of the character in question in the transformed versus untransformed plants. The risk of a transgene can thus be assessed under protected conditions and calibrated by the response of the untransformed plant under more natural conditions, as at least a first pass evaluation. It may be possible to manipulate either costs or benefits in order to contain the transgene. Benefit, which is quantified in the model as growth in the absence of competition, by definition is maximized for commercial return on the crop, and so may not be available for manipulation. Costs may therefore be the more likely target for manipulation: if you are going to have a magic helmet then you must have an Achilles heel. Some costs are a function of resistance itself, e.g., additional protein construction. It is not possible to have the transgene without these costs and the trade-off between costs and benefits in this case would be in choosing the transgene. As with any 'cost', its usefulness as a control will come in two parts: the extent to which it does not cut into commercial returns on the transgene; and the reliability of its genetic linkage to the transgene over time. The model delineates control possibilities in addition to costs inherent to the transgene. Costs most amenable to manipulation are included in two composite variables: seedling competitive ability (â) and seeds viable in the first season after seed set (Ő). â includes factors such as overtopping, and this is where construction costs would be taken into account. Ő is not simply seed set; Ő determines not just seedlings in year t + 1, but the character of the seed bank. Ő interacts with the behavior of seeds in the seed bank (germination fraction, persistence from season to season) to produce seedlings in future seasons. It is an essential part of the population persistence of a species that has a chance of producing few or no successful offspring in any one year. Ő, the number of seeds that make it through the first winter viable, is the first level of control for any novel allele. If Ő is low, then there are few viable seeds to remain in the seed bank and the capacity to get through bad years is severely limited. If Ĺ is high, then control shifts to the germination fraction Ĺ. If Ĺ is high, then, again, there are few seeds in the seed bank to get through bad years. If Ĺ is low, control shifts to S, the fraction of seeds that survive from one year to the next. Persistence over hard times is then limited by a low S. Pollen viability, ç, where male sterility manipulation may be applied, plays a similar role to Y. The equations of the model may be iterated numerically but Kelly et al. also provide a useful calculation for doubling time to assess the rate of spread of the transgene at early stages (when hemizygotes, individuals with only one copy of the transgene, comprise approximately less than 20% of the population). Although it requires the same information as the more detailed calculations that gave rise to the above conclusions, the doubling time equation is easier to calculate and provides a reasonable estimate of hemizygote spread. The focus on the natural population may seem to imply that the ecological interaction will be of importance only once the allele arrives there. In fact, it is likely that the ecological dynamic will have an effect at every step of the journey of the allele from crop to natural community. As introgression of the allele into the natural genome proceeds, the increment of protection that the allele offers will decrease in the context of the already well-protected wild genome; any cost it incurs is likely to have greater impact in a natural situation in which nutrient supplements become less and less available with increasing distance from the 'home' agricultural field. The model can be applied to determine this, by specifying the values of the factors that go in the applicable ratios. The goal of the model is to clarify the components and importance of fitness in the interaction between individuals with and without IR alleles in nature. The model may be usefully modified to include pathogens and seed predators that work either on the seed while it is still retained on the parent plant or in the seed bank. However, the model is also of significance to the basic ecology of sexually compatible invasive-native pairs where the active difference is in vulnerability to local herbivores, pathogens, or seed predators, whether the invasive individual is a crop, unwanted alien, or new mutation. "Source":[ http://www.checkbiotech.org/root/index.cfm?fuseaction=news&doc_id=11979&start=1&control=140&page_start=1&page_nr=101&pg=1].

---

Related articles

Antibiotic resistance countered - US scientists believe they may have found a way to stop the growing problem of bacteria becoming resistant to current drug treatments (11.7.2007)