Male-killing symbiosis across a predator-prey interaction: a genomic and experimental approach

大多数昆虫携带特殊的细菌，这些细菌永久地居住在它们的体内。这些细菌被称为共生体，对它们的昆虫宿主可能是有益的，也可能是代价高昂的。在这个项目中，我们将研究一种能够操纵宿主昆虫繁殖的细菌共生体--螺旋体。像许多昆虫共生体一样，螺旋体主要从雌性昆虫传播给它们的后代。由于雄性不能传播细菌，从共生体的角度来看，它们本质上是进化的“死胡同”。因此，共生体被选择来增加雌性后代的比例，结果是共生体诱导的生殖操纵策略的多样性。螺旋体采用了最直接的方法之一：携带共生体的雄性后代在成年前死亡(通常在卵子阶段)。这被认为对幸存的姐妹有利，尽管优势并不总是明确的。除了母体传播之外，共生体有时可以在不相关的个体之间传播，无论是在物种内部还是在物种之间。这种“水平”传播在进化上很重要，因为它是动物可遗传特征跨越物种边界的一条途径。然而，由于这种转移在性质上是罕见的，我们对这一过程知之甚少。在这个项目中，我们将研究两个密切相关的螺旋体群体，分别感染瓢虫和蚜虫，以了解当共生体在远缘昆虫之间移动时会发生什么。首先，我们将对13株螺旋体细菌的基因组进行测序：3株来自不同种类的瓢虫，10株来自豌豆蚜虫。豌豆蚜品系将包括导致雄性后代完全死亡、部分死亡和无死亡的细菌。我们将使用基因组来寻找螺旋体菌株之间在推测的杀死男性的遗传机制方面的相似和不同之处。我们的目标也是确定导致在蚜虫螺旋体中看到的杀死雄性的表型差异的遗传变异。在项目的第二部分，我们将重建一条可能的共生体传播路线。瓢虫是众所周知的蚜虫的捕食者，这种捕食者-猎物的关系在两个群体的进化历史中允许螺旋体在它们之间传播是非常有可能的。我们会向瓢虫注射含有杀死雄性的螺旋体的体液，反之亦然。我们将测试是否建立了一种新的可遗传感染，以及新感染是否会导致男性死亡。我们的研究结合了一个重建跨物种共生体传播的雄心勃勃的计划，并通过共生体基因组测序详细了解了共生体的传播机制。瓢虫和蚜虫提供了一个很好的机会来理解通过共生体的水平传播而获得的水平特征，并研究可能阻碍这种传递的障碍。通过在一个特定捕食者-猎物相互作用的背景下全面了解共生体的功能和传播，我们将为回答围绕新共生体的获取和共生体中介效应的进化的进一步问题铺平道路。昆虫是一个非凡的进化成功故事。隐藏在昆虫体内的细菌共生体在营养、防御和繁殖方面发挥着重要作用。因此，解释新的共生关系是如何以及为什么出现的，是理解昆虫进化的关键因素，也是我们提出的这个项目的首要目标。

Most insects carry specialised bacteria that dwell permanently inside them. These bacteria are known as symbionts and may be beneficial or costly to their insect host. In this project, we will investigate a bacterial symbiont, Spiroplasma, that is able to manipulate host insect reproduction to its own advantage.Like many insect symbionts, Spiroplasma is predominantly transmitted from female insects to their offspring. Because males cannot transmit the bacteria, they are essentially an evolutionary 'dead end' from the perspective of the symbiont. Symbionts are therefore under selection to increase the proportion of female offspring, and the result is a diversity of symbiont-induced reproductive manipulation strategies. Spiroplasma employs one of the most straightforward: male offspring carrying the symbiont die before adulthood (usually in the egg stage). This is assumed to benefit the surviving sisters, although the advantages are not always clear.In addition to maternal transmission, symbionts can occasionally be transmitted between unrelated individuals, either within or between species. This 'horizontal' transmission is evolutionarily important because it is a route for heritable characteristics to be passed across species boundaries in animals. However, since such transfers are rare in nature, we know little about the process. In this project, we will study two closely-related groups of Spiroplasma, infecting ladybirds and aphids respectively, in order to ask what happens when symbionts move between distantly-related insects.First, we will sequence the genomes of 13 strains of Spiroplasma bacteria: three from different species of ladybirds, and 10 from the pea aphid. The pea aphid strains will include bacteria causing complete, partial and no death of male offspring. We will use the genomes to look for similarities and differences among the Spiroplasma strains in the putative genetic mechanism for male-killing. We also aim to identify the genetic variation that drives the phenotypic differences in male-killing seen among the aphid Spiroplasma. In the second part of the project, we will recreate a likely route of symbiont transmission. Ladybirds are well-known as predators of aphids, and it is highly plausible that this predator-prey relationship allowed transmission of Spiroplasma between the two groups in their evolutionary past. We will carry out microinjection of body fluid containing male-killing Spiroplasma from aphids to ladybirds, and vice versa. We will test whether a new heritable infection is established and if the new infections cause male-killing. Our study combines an ambitious plan to recreate cross-species symbiont transmission with a detailed understanding of mechanism through symbiont genome sequencing. Ladybirds and aphids present a great opportunity to understand horizontal acquisition of traits via horizontal transmission of symbionts, and also to investigate the barriers that may stand in the way of that transfer. By gaining a full picture of symbiont function and transmission in the context of one specific predator-prey interaction, we will open the way to answer further questions surrounding the acquisition of novel symbionts, and the evolution of symbiont-mediated effects. Insects are an extraordinary evolutionary success story. Hidden inside insects, bacterial symbionts play important roles in nutrition, defence and reproduction. Explaining how and why new symbiotic associations arise is therefore a vital element for understanding insect evolution, and is the overarching aim of the project we propose.