Aphid secondary symbionts: from model system to crop pests

在温带国家，蚜虫是危害农作物最严重的害虫之一。它们之所以重要，一方面是因为它们造成直接损害，另一方面是因为它们是植物病毒性疾病的载体。传统上，蚜虫是通过大规模施用杀虫剂来控制的，但由于杀虫剂抗性水平的提高和公众要求减少使用农用化学品，这种控制措施的价值正在减弱。这使人们对其他形式的虫害防治重新产生兴趣，包括培养方法、抗性品种的开发和加强对天敌的控制。所有这些方法都需要对蚜虫生物学有更深入的了解，虽然人们普遍认为这一点已经非常众所周知，但过去五年的研究揭示了迄今为止意想不到的一系列与细菌的互惠相互作用，这些相互作用对蚜虫与宿主植物、天敌和非生物环境的相互作用方式有潜在的根本影响。这些细菌通常被称为次级共生体，以区别于所有蚜虫携带的初级共生体布氏菌。次生共生体已被发现影响蚜虫抵御寄生蜂和真菌病原体攻击的能力，以及抵抗热休克（短暂高温）的能力。但是，这些研究中的许多都涉及到单一的蚜虫和细菌菌株，并且已经有一些证据表明菌株之间存在差异。我们的第一个目标是通过对多个蚜虫菌株进行实验来探索共生体效应的重复性和特异性，我们系统地用多个菌株的共生体感染这些蚜虫菌株。目的是获得一个清晰的图像，蚜虫和细菌基因型如何贡献这些特征。我们的下一个目标是探索次生共生体的存在如何影响蚜虫种群动态，这在农业中显然很重要。我们将首先在实验室笼子实验中这样做，我们将利用我们第一次实验的结果，在存在或不存在拟寄生虫的情况下，建立受感染和未受感染蚜虫克隆的不同组合。我们预测在天敌存在的情况下，次生共生体会影响蚜虫的数量和克隆组成。为了理解和分析这些实验，我们将发展数学种群模型，这些模型在分析其他种群笼实验时已被证明是有用的。我们还将在实地环境中进行人口实验，以进一步增加现实性。豌豆蚜虫是一种非常有价值的模式生物，也是一种次要的农业害虫。但是，在这个系统中获得的结果在多大程度上适用于主要害虫，如谷物蚜虫？我们将调查两种谷物蚜虫，以确定它们与次生共生体的关联频率，然后测试共生体是否影响其宿主抵抗寄生虫和真菌病原体的攻击以及抵抗热休克的能力。农民越来越多地维持田地边缘和其他栖息地，为害虫的天敌提供替代猎物或宿主。因此，害虫和田间边缘物种的种群动态通过共同的天敌联系在一起（群落生态学家对这种相互作用非常感兴趣）。我们的最终目标是通过实验探索次生共生体的存在如何影响这种关系，这种关系可能会增强或削弱害虫管理。我们希望我们的工作能够为害虫管理和可持续农业方法的科学基础做出贡献，从而提高农民的收入，为英国的生活质量做出贡献

Aphids are one of the most serious pests attacking crops in temperate countries. They are important both because of the direct damage they cause and also because they are vectors of plant viral diseases. Traditionally, aphids have been controlled by the large-scale application of insecticides, but the value of this control measure is being weakened by increasing levels of insecticide resistance and by public demand for decreased use of agriculture chemicals. This has led to renewed interest in other forms of pest control including cultural methods, the development of resistant varieties and enhancing natural enemy control. All these methods require a greater understanding of aphid biology, and while it is widely perceived that this is already very well-known, research in the last five years has revealed a hitherto unexpected series of mutualistic interactions with bacteria that have potentially radical effects on the way aphids interact with their host plants, natural enemies and the abiotic environment. These bacteria are generally referred to as secondary symbionts to distinguish them from the primary symbiont Buchnera that all aphids carry. Secondary symbionts have been found to affect an aphids ability to withstand attack by parasitoid wasps and fungal pathogens, and to resist heat shock (transient high temperatures). But many of these studies have involved single strains of aphids and bacteria, and there is already some evidence for variation across strains. Our first aim is to explore the repeatability and specificity of the effects of the symbionts by experimenting with multiple aphid strains which we systematically infect with multiple strains of symbiont. The objective is to gain a clear picture of how both the aphid and the bacterial genotypes contribute to these characteristics. Our next objective is to explore how the presence of secondary symbionts affects aphid population dynamics, clearly of importance in agriculture. We shall do this first in laboratory cage experiments in which we shall make use of the results of our first experiments to set up different combination of infected and uninfected aphid clones, in the presence or absence of parasitoids. We predict that both aphid numbers and clonal composition will be influenced by secondary symbionts in the presence of natural enemies. To understand and analyse the experiments we shall develop mathematical population models that have already proved useful to us in analysing other population cage experiments. We shall also carry out population experiments in a field setting, to add further realism. The pea aphid is a very valuable model organism, and a minor agricultural pest. But how far do results obtained in this system apply to major pests, such as cereal aphids? We shall survey two cereal aphid species to determine the frequency with which they are associated with secondary symbionts, and then test whether the symbionts affect their hosts' ability to resist parasitoid and fungal pathogen attack, and to combat heat shock. Farmers increasingly maintain field margins and other habitats that support alternative prey or hosts for the natural enemies of pests. The population dynamics of the pest and the field margin species are thus linked by the shared natural enemy (an interaction of great interest to community ecologists). Our final objective is to explore experimentally how the presence of secondary symbionts may affect this relationship, something that might either enhance or detract from pest management. We hope our work will contribute to the science underpinning pest management and sustainable agricultural methods, thus improving farmers' incomes and contributing to the quality of life in the UK