Why does Drosophila vary in susceptibility to parasitoid wasps?

这笔赠款的目的是了解为什么人群中的个人对感染的易感性不同。这种变异决定了疾病的负担，并允许种群进化出抵抗力。对于研究人员来说，它可以提供对宿主-寄生虫共同进化和免疫系统功能的洞察。我们将首先使用高通量基因组测序来鉴定使果蝇对其最重要的天敌--寄生蜂具有抵抗力的遗传变异。这些结果将通过产生基因完全相同的苍蝇来验证，这些苍蝇只是不同的变种。接下来，我们将识别这些基因如何使个体产生抵抗力。血细胞是负责杀死寄生虫卵的免疫细胞。在抗性果蝇中，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，我们发现了一种名为Sit的基因的多态性，该基因与抗药性密切相关。Sit由神经元分泌，控制细胞迁移。我们将检验这一假说，即抗性是由外周神经元中Sit的表达变化引起的，从而使血细胞离开固着簇并进入循环，从而产生一种结构性激活的免疫系统，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在人群中保持不变，因为耐药等位基因成本高昂，但这些成本的原因却鲜为人知。我们将研究成本是否源于结构性激活的免疫系统造成的附带损害。对寄生蜂具有抗药性的果蝇的适合度显著下降，我们将使用除了控制抗药性的变种外在基因上相同的果蝇来确定哪些抗药性基因是昂贵的。为了测试细胞免疫防御是否昂贵，我们可以使用基因工具去除这些果蝇的血细胞，并检查这是否降低了成本。为了测试是否将血细胞释放到循环中是代价高昂的，我们将使用基因技术将抗药性果蝇的血细胞重新放入定位簇，并将易感果蝇的血细胞释放到循环中，然后测量对健康的影响。遗传抗性有时对广泛的寄生虫有效，但往往具有高度的特异性。这种特异性在昆虫等生物体中令人惊讶，因为机械学研究发现，它们的先天免疫系统辨别能力有限。为了解决这一悖论，我们将测试一种假设，即改变先天免疫反应大小的基因提供广谱抵抗，而克服破坏免疫的寄生虫因素的基因提供特定抵抗。对寄生蜂的抗性既涉及广谱抗性基因，也涉及特异性抗性基因。通过在实验室中跟踪种群进化抗药性时的基因频率，我们将识别这些基因，并使用基因工程果蝇来证实结果，这些果蝇除了有问题的变异外，其他都是相同的。这些苍蝇将使我们能够检验我们的预测，即广谱抵抗力是由于先天免疫反应(循环血细胞数量)的增加而产生的，而特定抵抗力则取决于血细胞对使免疫系统失效的黄蜂毒素的敏感性。

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.