Aphids, Bacteria and Fungal Pathogens; the Ecology of a Complex Symbiosis

在过去的二十年里，分子技术的简单可用性彻底改变了我们对动植物与微生物相互作用的理解。许多以前未被怀疑的共生现象已经被发现，我们对生物多样性和群落生态学的理解也出现了许多新的问题。这项建议是一个长期项目的一部分，以了解一个复杂的互惠共生关系的动态和持久性，涉及一个蚜虫和不少于八个细菌的合作伙伴。蚜虫是温带地区常见的昆虫，也是这些地区最重要的农作物害虫。尽管自科学时代开始以来一直在进行密切的研究，但在过去的15年里，随着细菌共生体的重要性变得明显，对它们生物学的许多方面进行了全面的重新评估。蚜虫以营养不平衡的植物汁液为食，50年来人们已经知道它们携带一种称为Buchnera的专性（或初级）细菌共生体，该共生体合成它们饮食中缺少的必需营养素。但我们现在知道，除了Buchnera之外，至少还有其他七种兼性或次级共生体，存在于一些蚜虫中，而不是其他蚜虫。此外，这些兼性共生体对寄主的生物学特性有许多影响，包括赋予寄主对寄生虫和病原菌的抗性，使寄主能够承受热激或利用不同的寄主植物，以及影响生活史策略。我们实验室特别研究了共生体如何影响蚜虫抵御真菌病原菌攻击的能力。我们发现，一种名为Regiella insecticola的细菌显著增强了对最常见真菌疾病的抵抗力。奇怪的是，这种共生体物种特别与以三叶草为食的蚜虫有关（豌豆蚜虫具有复杂的种群结构，由与豌豆家族中不同宿主植物相关的遗传分化的“生物型”组成）。最近，我们的实验室绘制了Regiella的遗传结构图，显示来自豌豆蚜虫的分离物被组织成两个主要的遗传群（或分支）。我们还发现，其他最近发现的次级共生体可以赋予真菌抗性，这表明这可能是一个一般的策略，共生体用于通过主机population.The建议是支持继续在我们的实验室豌豆蚜虫共生体专注于豌豆蚜虫，他们的寄主植物和真菌病原体之间的相互作用的生态，和兼性共生体赋予抗性。我们将测试的假设，蚜虫生物型三叶草遭受特别是从真菌病原体，因此需要携带Regiella。使用细菌的相互介绍，我们将问为什么Regiella从两个主要的分支感染不同的生物型，并探讨是否所有的分支提供电阻。真菌仍然可以杀死携带Regiella的蚜虫（尽管概率较低），但它们不太可能产生感染性孢子，我们将测试这是通过亲缘选择进化而来的假设。迄今为止，所有的工作都涉及一种真菌病原体，尽管我们从社区研究中知道，该领域还存在其他病原体。我们将研究真菌耐药的特异性。我们将在最近的试点研究的基础上，证明其他共生体的一些分离株具有耐药性，以确定这种情况在自然种群中发生的程度。最后，我们将使用现代DNA测序技术来检验真菌耐药性是由这些不相关细菌之间水平转移的共同机制引起的假设。

The easy availability of molecular techniques over the last two decades has revolutionised our understanding of how animals and plants interact with micro-organisms. Many previously unsuspected symbioses have been discovered and many new issues for our understanding of biodiversity and community ecology have arisen. This proposal is part of a long-term project to understand the dynamics and persistence of a complex mutualistic symbiosis involving an aphid and no less than eight bacterial partners. This has become a model system used by laboratories around the world exploring the ecology and evolution of symbiosis.Aphids are familiar insects in temperate regions and in these areas are the most important pests of arable crops. Despite being closely studied since the dawn of the scientific age the last 15 years have seen a complete reappraisal of many aspects of their biology as the importance of their bacterial symbionts has become apparent. Aphids feed on plant sap which is nutritionally imbalanced and it has been known for 50 years that they carry an obligate (or primary) bacterial symbiont called Buchnera which synthesises essential nutrients missing in their diet. But we now know that in addition to Buchnera there are at least seven other facultative or secondary symbionts, present in some aphids but not others. Moreover, these facultative symbionts have many effects on their host's biology including conferring resistance to parasites and pathogens, enabling their hosts to withstand heat shock or use different host plants, and influencing life history strategy.Our laboratory has particularly worked on how symbionts influence the aphid's ability to withstand attack by fungal pathogens. We discovered that one bacteria called Regiella insecticola markedly increased resistance to the most common fungal disease. Curiously this symbiont species is particularly associated with aphids feeding on clover (pea aphid has a complex population structure comprised of genetically differentiated "biotypes" associated with different host plants within the pea family). Very recently our laboratory has mapped the genetic structure of Regiella showing that isolates from pea aphid are organised into two major genetic groups (or clades). We have also found that other recently discovered secondary symbionts can impart fungal resistance which suggests that this might be a general strategy that symbionts use to spread through host populations.The proposal is to support continuing work on pea aphid symbionts in our laboratory focussing on the ecology of the interaction between pea aphid, their host plants and fungal pathogens, and the facultative symbionts that confer resistance. We shall test the hypothesis that the aphid biotype on clover suffers particularly from fungal pathogens and hence needs to carry Regiella. Using reciprocal introductions of bacteria we shall ask why Regiella from the two major clades infect different biotypes and explore whether all clades provide resistance. Fungus can still kill aphids carrying Regiella (though with lower probability) but they are then less likely to produce infectious spores and we shall test the hypothesis that this has evolved through kin selection. All work to date has involved a single fungal pathogen though we know from our community studies that other pathogens are present in the field. We shall investigate the specificity of fungal resistance. We shall build on our recent pilot study demonstrating that some isolates of other symbionts confer resistance to establish the extent to which this occurs in natural populations. Finally, we shall use modern DNA sequencing techniques to test the hypothesis that fungal resistance is caused by a common mechanism that has been transferred horizontally amongst these unrelated bacteria.