Resistance: The role of genetic architecture and refuge strategy on the evolution of resistance to Bt-crops in lepidopteran pests

bt作物是一种有效的控制农业害虫，特别是蛾毛虫的方法，其基因工程可以表达多种源自苏云金杆菌的毒素。然而，与传统杀虫剂一样，全球一些昆虫种群已经找到了进化出抗Bt机制的方法。因此，Bt作物的可持续利用取决于防止或至少大大减缓抗性进化的速度，以及通过开发针对害虫防御的一个或多个弱点的新的Bt作物品种。当抗性性状是完全隐性的（而不是部分隐性或显性的），以及当群体中易感个体的比例很高（这阻止了抗性等位基因的两个拷贝在同一个体中聚集）时，抗性在群体中的传播要慢得多。这就是为什么bt作物被设计成提供高剂量的毒素，应该立即杀死所有的个体，并且它们与非bt植物一起种植，以维持足够多的易感个体。问题是，由于各种原因，这种抗性管理策略所基于的假设很少适用于现场条件。本项目通过研究玉米两种主要害虫的抗Bt基因，将这些假设置于显微镜下，并将这些信息纳入预测模型，为设计抗虫管理策略提供更细致入微的基础。主要的焦点物种是非洲玉米跟踪虫，这是撒哈拉以南非洲玉米和高粱的主要害虫。该项目旨在发现南非非洲玉米跟踪虫种群中发生的遗传变化，这些变化赋予了对Bt玉米的显性抗性，这种抗性导致了经济上的失败和对原始Bt作物品种的替代。对遗传多样性的调查还将使我们能够评估东非国家bt抗性进化的风险，那里的bt玉米即将被释放。次要焦点物种是秋粘虫，它是美洲玉米和棉花的主要害虫，在那里它已经进化出对某些Bt毒素的抗性。该物种于2016年在西非和中非定居，现在已蔓延到非洲东部和南部，在那里引发了玉米田的紧急大规模农药喷洒。通过建立南非秋粘虫种群对目前使用的bt玉米的耐受性（及其遗传基础），并将这些信息输入到我们的模型中，我们将及时评估这种非本地害虫物种如何在新的生态环境中应对当前的bt抗性管理制度。这项研究的好处是，有关Bt抗性和耐受机制的遗传特性和多样性的具体知识，当与更现实的模型结合在一起时，将有助于在替代避难所设计方法下对抗性的出现和传播速度做出更可靠的预测。研究结果，包括对两种主要害虫的比较分析，也将有助于开发新的Bt作物，提供长期控制鳞翅目害虫的方法。最后，产出将提供一般的科学见解，并与解决迫切需要解决的区域粮食安全问题直接相关。

Bt-crops, engineered to express a variety of toxins derived from Bacillus thuringensis, are an efficient method for controlling agricultural insect pests, particularly moth caterpillars. However, as with conventional insecticides, several insect populations around the globe have found ways to evolve mechanisms of resistance to Bt. Therefore, the sustainable use of Bt-crops is dependent on preventing, or at least greatly slowing, the rate at which resistance evolves, and by developing new Bt-crop varieties that target one or more weak points in the pest defences.Resistance spreads through a population much more slowly when the resistance trait is fully recessive (as opposed to partially recessive or dominant), and when the proportion of susceptible individuals in the population is high (which prevents two copies of the resistance alleles coming together in the same individual). This is why Bt-crops are engineered to deliver a high dose of toxin that is supposed to kill all individuals outright, and they are grown together with non-Bt plants, to sustain a sufficiently large number of susceptible individuals. The problem is that, for various reasons, the assumptions on which this resistance management strategy is based rarely apply to field conditions. This project is about putting these assumptions under the microscope by studying the genetics of Bt resistance in two major moth pests of maize, and incorporating this information into a predictive model to provide a more nuanced basis for designing insect resistance management strategies.The primary focal species is the African maize stalkborer, the major insect pest of maize and sorghum in sub-Saharan Africa. This project aims to discover the genetic changes that have occurred in the African maize stalkborer population in South Africa to confer dominant resistance to Bt maize, which led to the economic failure and replacement of the original Bt-crop variety. Surveys of genetic diversity will also allow us to assess the risk of Bt-resistance evolving in east African countries, where Bt-maize is close to being released.The secondary focal species is the Fall armyworm, a major pest of maize and cotton in the Americas, where it has evolved resistance to some Bt toxins. This species colonised west and central Africa in 2016, and has now spread to eastern and southern Africa, where it has elicited emergency large scale pesticide spraying of maize fields. By establishing the degree of tolerance (and its genetic basis) in the South African Fall armyworm population to the Bt-maize currently in use, and plugging this information into our model, we will provide a timely evaluation of how this non-native pest species is likely to cope with the current Bt-resistance management regime in a new ecological setting.The benefits of this research are that specific knowledge about the genetic identity and diversity of Bt resistance and tolerance mechanisms, when put together with a more realistic model, will aid in making more reliable predictions about the rate of appearance and spread of resistance under alternative refuge design approaches. The results, including comparative analysis of two major pest species, will also contribute to developing new Bt crops that provide longer-term lepidopteran pest control. Finally, the outputs will both provide general scientific insights and be directly relevant to addressing a regional food security problem urgently in need of solutions.