Department of Zoology, University of Wisconsin, Madison, Wisconsin 53706, USA.
Ecol Appl. 2011 Mar;21(2):503-15. doi: 10.1890/09-1869.1.
Pyramid transgenic crops that express two Bacillus thuringiensis (Bt) toxins hold great potential for reducing insect damage and slowing the evolution of resistance to the toxins. Here, we analyzed a suite of models for pyramid Bt crops to illustrate factors that should be considered when implementing the high dose-refuge strategy for resistance management; this strategy involves the high expression of toxins in Bt plants and use of non-Bt plants as refuges. Although resistance evolution to pyramid Bt varieties should in general be slower, resistance to pyramid Bt varieties is nonetheless driven by the same evolutionary processes as single Bt-toxin varieties. The main advantage of pyramid varieties is the low survival of insects heterozygous for resistance alleles. We show that there are two modes of resistance evolution. When populations of purely susceptible insects persist, leading to density dependence, the speed of resistance evolution changes slowly with the proportion of refuges. However, once the proportion of non-Bt plants crosses the threshold below which a susceptible population cannot persist, the speed of resistance evolution increases rapidly. This suggests that adaptive management be used to guarantee persistence of susceptible populations. We compared the use of seed mixtures in which Bt and non-Bt plants are sown in the same fields to the use of spatial refuges. As found for single Bt varieties, seed mixtures can speed resistance evolution if larvae move among plants. Devising optimal management plans for deploying spatial refuges is difficult because they depend on crop rotation patterns, whether males or females have limited dispersal, and other characteristics. Nonetheless, the effects of spatial refuges on resistance evolution can be understood by considering the three mechanisms determining the rate of resistance evolution: the force of selection (the proportion of insects killed by Bt), assortative mating (deviations of the proportion of heterozygotes from Hardy-Weinberg equilibrium at the total population level), and male mating success (when males carrying resistance alleles find fewer mates). Of these three, assortative mating is often the least important, even though this mechanism is the most frequently cited explanation for the efficacy of the high dose-refuge strategy.
表达两种苏云金芽孢杆菌（Bt）毒素的金字塔转基因作物具有很大的潜力，可以减少昆虫的损害，并减缓对这些毒素的抗性的进化。在这里，我们分析了一系列的金字塔 Bt 作物模型，以说明在实施高剂量避难所策略来进行抗性管理时应考虑的因素；该策略涉及 Bt 植物中毒素的高表达和使用非 Bt 植物作为避难所。虽然对金字塔 Bt 品种的抗性进化总体上应该较慢，但对金字塔 Bt 品种的抗性仍然是由与单一 Bt 毒素品种相同的进化过程驱动的。金字塔品种的主要优势是对抗性等位基因杂合的昆虫的低生存能力。我们表明，存在两种抗性进化模式。当纯粹易感昆虫种群持续存在，导致密度依赖性时，抗性进化的速度随避难所比例的变化而缓慢变化。然而，一旦非 Bt 植物的比例低于易感种群无法持续存在的阈值，抗性进化的速度就会迅速增加。这表明需要采用适应性管理来保证易感种群的持续存在。我们比较了在同一田间播种 Bt 和非 Bt 植物的种子混合物的使用和空间避难所的使用。与单一 Bt 品种一样，如果幼虫在植物之间移动，种子混合物可以加速抗性进化。设计部署空间避难所的最佳管理计划是困难的，因为它们取决于作物轮作模式、雄性或雌性的扩散能力以及其他特征。尽管如此，通过考虑决定抗性进化速度的三个机制，可以理解空间避难所对抗性进化的影响：选择力（被 Bt 杀死的昆虫比例）、交配选择（在总体种群水平上杂合子比例偏离哈迪-温伯格平衡）和雄性交配成功率（携带抗性等位基因的雄性找到的配偶较少时）。在这三个机制中，交配选择通常是最重要的，即使这一机制是高剂量避难所策略有效性的最常被引用的解释。
