Aphid secondary symbionts: a eukaryote horizontal gene pool

蚜虫是大多数温带陆地生态系统的重要成员，包括一些最重要的农业和林业害虫。人们早就知道，几乎所有的蚜虫都携带着一种共生细菌，这种细菌提供了必需的氨基酸和其他营养物质，而这些在蚜虫相当贫乏的饮食中是不存在的。最近，人们发现，大多数蚜虫还藏匿着一种或多种其他细菌，这些细菌对它们的适合性和表现有各种影响。因为这些细菌并不存在于每一种蚜虫中，它们被称为次生共生体。最近的研究表明，次生共生体除了可以提高蚜虫对寄生蜂和病原菌的抵抗力外，还可以帮助蚜虫抵御热激，并影响寄主植物的利用。次生共生体构成了一个基因库，可以帮助蚜虫应对不同的环境挑战。由于次生共生体可以在蚜虫克隆和物种之间移动，它们可以被称为水平基因库，这是我们从细菌遗传学中借用的一个术语，在细菌遗传学中，质粒(可以在细菌之间移动的染色体外元素)具有类似的作用。虽然水平基因库长期以来一直被认为对细菌及其近亲很重要，但它们对真核生物(包括植物和动物在内的有核生物)的重要性却远不清楚。这个项目的工作试图了解次生共生体群落的结构：是什么决定了共生体的多样性和种群结构，以及什么影响了它的进化稳定性。它由三个子项目组成。首先是开发一个数学模型，综合我们对次生共生体动力学的所有了解，并确定最关键的研究领域。目前还不存在这样的模型，定量方法对于分析这类复杂的非线性相互作用至关重要。第二个项目是试验性的，试图提供有关蚜虫-次生共生体相互作用的部分信息，目前我们在这方面的知识存在很大差距。这就是共生菌对蚜虫有性世代的影响，父本传播的频率，以及两种共生菌联合感染的后果。该项目的最后部分试图使用尖端细菌遗传菌株分型来描述次生共生体的结构，并测试关于它是如何构成的假说。

Aphids are important members of most temperate terrestrial ecosystems, and include some of the most significant agricultural and forestry pests. It has long been known that nearly all aphids carry a symbiotic bacterium that provides essential amino acids and other nutrients that are absent in the aphids' rather depauperate diet. More recently it has been discovered that most aphids also harbour one or more of a series of other bacteria that have a variety of effects on their fitness and performance. Because these bacteria are not present in every aphid they are called secondary symbionts. Recent research has shown that secondary symbionts can, among other things, increase aphid resistance to parasitic wasps and pathogenic fungi, help the aphid withstand heat shock, and influence host plant use. Secondary symbionts constitute a pool of genes that can help the aphid cope with different environmental challenges. Because secondary symbionts can move between aphid clones and species they can be called a horizontal gene pool, a term we borrow from bacterial genetics where plasmids (extra-chromosomal elements that can move between bacteria) have an analogous role. Though horizontal gene pools have long been recognised as important for bacteria and their relatives, their significance for eukaryotes (organisms with nuclei including plants and animals) is far less clear. The work in this project seeks to understand how the community of secondary symbionts is structured: what determines the diversity and population structure of the symbionts, and what affects its evolutionary stability. It consists of three subprojects. The first is to develop a mathematical model to synthesise all we know about secondary symbiont dynamics and to identify the most critical areas for research. No such model exists at the moment, and quantitative approaches are essential to analyse complex non-linear interactions such as these. The second project is experimental and seeks to provide information about a part of the aphid-secondary symbiont interaction where we currently have large gaps in our knowledge. This is the effect of symbionts on the aphid sexual generation, the frequency of paternal transmission, and the consequences of joint infection by two symbionts. The final part of the project seeks to use cutting edge bacterial genetic strain typing to describe the structure of the secondary symbiont community and to test hypotheses about how it is structured.