Why does Drosophila vary in susceptibility to parasitoid wasps?

这项资助的目的是了解为什么人群中的个体对感染的易感性各不相同。这种变异决定了疾病的负担，并使种群能够进化出抗性。对于研究者来说，它可以为宿主-寄生虫的共同进化和免疫系统的功能提供见解。我们将开始通过使用高通量基因组测序，以确定遗传变异，使果蝇黑腹果蝇抵抗其最重要的天敌，寄生蜂。这些结果将通过产生基因相同的果蝇来验证，这些果蝇只是在所讨论的变体上有所不同。我们接下来将确定这些基因是如何使个体产生抗性的。血细胞是负责杀死寄生虫卵的免疫细胞。在有抵抗力的果蝇中，我们发现血细胞已经从与神经元相关的固着簇移动到循环中。此外，在我们的初步数据中，我们发现了一个名为slit的基因的多态性，该基因与抗性密切相关。Slit由神经元分泌并控制细胞迁移。我们将测试这一假设，即抗性是由外周神经元中Slit的表达变化引起的，使得血细胞离开固着簇并进入循环，从而导致组成性激活的免疫系统，可以迅速杀死入侵的寄生虫。感染易感性的遗传变异通常被认为是在人群中保持的，因为耐药等位基因是昂贵的，但这些成本的原因知之甚少。我们将研究是否成本造成的附带损害所造成的组成性激活的免疫系统。对拟寄生物具有抗性的苍蝇的适应性明显降低，我们将使用除了控制抗性的变体之外遗传上相同的苍蝇来确定哪些抗性基因是昂贵的。为了测试细胞免疫防御是否是昂贵的，我们可以使用遗传工具从这些果蝇中去除血细胞，并检查这是否消除了成本。为了测试是否是血细胞释放到循环中的代价高昂，我们将使用遗传技术将抗性苍蝇的血细胞返回到固着簇中，并将易感苍蝇的血细胞释放到循环中，然后测量对适应性的影响。遗传抗性有时对广泛的寄生虫有效，但通常是高度特异性的。这种特异性在昆虫等生物体中令人惊讶，因为机械研究发现它们的先天免疫系统具有有限的识别能力。为了解决这一矛盾，我们将测试的假设，即基因改变的先天免疫反应的大小提供广谱抗性，而基因克服寄生虫因素破坏免疫提供特定的阻力。对寄生蜂的抗性涉及广谱抗性基因和特异抗性基因。通过跟踪种群在实验室中进化抗性时的基因型频率，我们将识别这些基因，并使用除了所讨论的变体之外完全相同的基因工程果蝇来确认结果。这些果蝇将使我们能够测试我们的预测，即广谱抗性是由先天免疫反应（循环血细胞数量）的增加引起的，而特异性抗性取决于血细胞对使免疫系统失效的黄蜂毒液的易感性。

The aim of this grant is to understand why individuals within populations vary in their susceptibility to infection. This variation determines the burden of disease and allows populations to evolve resistance. For reseatchers, it can provide insights into host-parasite coevolution and functioning of immune systems. We will begin by using high-throughput genome sequencing to identify the genetic variants that make the fruit fly Drosophila melanogaster resistant to its most important natural enemy, parasitoid wasps. These results will be verified by generating genetically identical flies that only differ in the variants in question.We will next identify how these genes make individuals resistant. Hemocytes (blood cells) are the immune cells responsible for killing the parasitoid egg. In resistant flies we have found that hemocytes have moved from sessile clusters associated with neurons into circulation. Furthermore, in our preliminary data we have found a polymorphism in a gene called slit that is strongly associated with resistance. Slit is secreted by neurons and controls cell migration. We will test the hypothesis that resistance is caused by the expression of Slit changing in peripheral neurons so that hemocytes leave sessile clusters and move into circulation, resulting in a constitutively activated immune system that can rapidly kill invading parasitoids. Genetic variation in susceptibility to infection is often thought to be maintained in populations because resistant alleles are costly, but the causes of these costs are poorly understood. We will examine whether costs result from collateral damage caused by a constitutively activated immune system. Flies that are resistant to parasitoids suffer a marked reduction in fitness, and we will use flies that are genetically identical apart from the variants controlling resistance to identify which resistance genes are costly. To test whether it is cellular immune defences that are costly, we can use genetic tools to remove the hemocytes from these flies and examine whether this removes the costs. To test whether it is the release of hemocytes into circulation that is costly, we will use genetic techniques to return the hemocytes of resistant flies to sessile clusters and release the hemocytes of susceptible flies into circulation, and then measure the effect on fitness.Genetic resistance is sometimes effective against a broad range of parasites, but is often highly specific. This specificity is surprising in organisms like insects, as mechanistic studies have found that their innate immune system has limited discriminatory power. To resolve this paradox we will test the hypothesis that genes that alter the magnitude of the innate immune response provide broad-spectrum resistance, while genes that overcome parasite factors sabotaging immunity provide specific resistance. Resistance to parasitoids involves both broad-spectrum and specific resistance genes. By tracking genotype frequencies as populations evolve resistance in the lab, we will identify these genes and confirm the results using genetically engineering flies that are identical except for the variants in question. These flies will allow us to test our prediction that broad-spectrum resistance results from an increase in the magnitude of the innate immune response (number of circulating hemocytes), while specific resistance depends on the susceptibility of hemocytes to wasp venoms that disable the immune system.