Resistance: DNA methylation and the evolution of pesticide-resistance genes in aphids

大多数昆虫物种都是专门的寄生虫，它们适应于在一个或几个密切相关的植物物种上定居。大约10%的昆虫食草动物是多面手(多食性)，这些往往是最有害的害虫，已经对许多杀虫剂产生了抗药性。多面手昆虫容易出现这种“破坏杀虫剂”的情况，因为它们暴露在广泛的宿主环境中，已经对许多不同的植物化学物质产生了抗药性。令人担忧的是，这些植物化学物质中的许多都被用来衍生杀虫剂，这使得这些多面手昆虫预先适应了杀虫剂的破坏。桃长管蚜(Myzus persicae，GPA)可在400多种植物上定殖，50年来已对71种化学物质产生抗性或耐受性。GPA已成为理解“农药破坏”如何演变的典范。在BBSRC以前资助的一个项目中，我们发现了支撑昆虫显著适应潜力的分子机制；两个基因(DNMT3A和3B)的DNA甲基化使GPA能够适应不同的植物物种，并在没有遗传修饰的情况下克服农药毒性。基因相同的克隆可以适应新的寄主植物，并在暴露后几小时或几天内表现出杀虫剂抗药性，而且它们(基因相同的)后代变得越来越适应。我们已经证明解毒基因的表达是由DNA甲基化(这是一个表观遗传过程)控制的。我们还发现，当DNMT3A/B甲基化被破坏时，蚜虫就不能再进行调整了。我们还知道，DNMT3A/B甲基化对不同基因的上调或下调取决于所遇到的寄主植物或农药。在此基础上，我们的新建议有两个主要目标：1)识别和描述由DNMT3A/B介导的从头DNA甲基化影响的整个基因网络。这一点很重要，因为这些基因使蚜虫能够检测和响应杀虫剂，因此，这将有助于开发针对新的昆虫靶标的杀虫剂。因此，我们的项目合作伙伴先正达和其他公司将在此类杀虫剂的开发方面获得帮助。2)了解在DNMT3A/B途径中这些“抗性基因”的遗传变异所依据的DNA变异产生和形成的进化力量。这一点很重要，因为50年的杀虫剂使用将在蚜虫(和其他害虫)的基因组中留下可识别的签名。通过研究这一信号，我们可以优化害虫的控制策略。假设：1)GPA拥有帮助昆虫检测新的寄主植物/杀虫剂的基因，从而诱导DNMT3A/B上调。2)DNMT3A/B甲基化基因使昆虫能够解毒这些化学物质。3)通用蚜虫(如GPA)的DNMT3A/B表达水平在寄主切换和农药暴露后的变化幅度大于专门化蚜虫(如甘蓝蚜和麦长管蚜)。4)受DNMT3A/B影响的协同调控网络对通用型蚜虫的影响比专化型蚜虫更广泛。5)通用害虫(包括食草动物、动物和人类害虫)表现出不同于专业害虫的基因组流线型、谱系特异性基因家族和基因重复。6)农药处理的历史使用将影响5种进化力量(突变、重组、基因流动、遗传漂移和自然选择)，这些力量塑造了世界各地种群GPA的基因组变异。与我们的项目合作伙伴先正达合作，我们设计了三个令人兴奋的实验来验证这些假设。我们相信，这项研究产生的知识可能会发现昆虫控制的新目标，并将有助于优化特定物种的昆虫控制策略，从而确保可持续农业。

Most insect species are specialist parasites that have adapted to colonize one or a few closely related plant species. Circa 10% of all insect herbivores are generalist ("polyphagous"), and these are often the most noxious pests, having evolved resistance to many pesticides. Generalist insects are prone to such "pesticide-breaking" because by being exposed to a wide range of hosts, they have already evolved resistance to many different plant chemicals. Worryingly, many of these phytochemicals have been used to derive pesticides, and this makes such generalist insects pre-adapted to pesticide-breaking. The green peach aphid (GPA) Myzus persicae can colonize over 400 different plant species, and it has evolved resistance or tolerance to 71 chemicals in 50 years. GPA has become a model to understand how "pesticide-breaking" has evolved. In a former BBSRC funded project, we discovered the molecular mechanism underpinning the insect's remarkable adaptive potential; DNA methylation by two genes (DNMT3A and 3B) enables GPA to adjust to diverse plant species and overcome pesticide toxicity without genetic modification. Genetically identical clones can adjust to a new host plant and show pesticide resistance within hours or days after exposure, and furthermore, their (genetically identical) offspring becomes increasingly well-adjusted. We have shown that the expression of detoxification genes is controlled by DNA methylation (which is an epigenetic process). We also showed that when DNMT3A/B methylation is knocked-down, that the aphids can no longer adjust. We also know that different genes are being up- or down-regulated by DNMT3A/B methylation depending on the host plant or pesticide being encountered. Building on this knowledge, our new proposal has two principal objectives: 1) Identify and describe the entire gene networks affected by de novo DNA methylation mediated by DNMT3A/B. This is important because these are the genes that enable the aphid to detect and respond to the pesticide, and hence, this will help the development of pesticides against novel insect targets. Our Project Partner Syngenta, and others, will thus be helped in their development of such pesticides. 2) Understand the evolutionary forces that generate and shape the DNA variation underpinning the genetic variation in these "resistance genes" in the DNMT3A/B pathway. This is important because the 50 years of pesticide usage will have left a decipherable signature in the genome of the aphids (and other pest insects). By studying this signature, we can optimise pest insect control strategies.The hypotheses are:1) GPA possesses genes that help the insect to detect the novel host plant / pesticides, which instigates DNMT3A/B upregulation.2) The DNMT3A/B methylated genes enable the insect to detoxify these chemicals. 3) Generalist aphids (e.g., GPA) display a larger change in DNMT3A/B expression levels upon host switch and pesticide exposure than specialist aphids (such as the cabbage aphid Brevicoryne brassicae, and the English grain aphid Sitobion avenae).4) Co-regulatory networks affected by DNMT3A/B are more extensive for generalist than specialist aphids. 5) Generalist insect pests (including herbivores, animal and human insect pests) show genome streamlining, lineage-specific gene families and gene duplication that is distinctive from specialist pest insects. 6) The historic use of pesticide treatment will have impacted the 5 evolutionary forces (mutation, recombination, gene flow, genetic drift and natural selection), which has shaped the genomic variation in GPA in populations across the world. In collaboration with our Project Partner Syngenta, we have designed three exciting experiments that test these hypotheses. We believe the knowledge generated by this research is likely to uncover new targets for insect control, and will help to optimise species-specific insect control strategies, and hence secure sustainable agriculture.