Understanding the genetic mechanisms of phenotypic plasticity in insect migration

每年有数十亿只昆虫迁徙数千公里，寻找食物、庇护所和繁殖的地方。要踏上如此艰难的旅程，需要由多个基因编码的生理、行为和形态适应。对于许多昆虫来说，迁徙是在预计栖息地恶化的情况下进行的，从而使种群能够在面临环境压力的情况下取得成功。这种在一代人中对环境线索做出反应的能力需要灵活的遗传基础，这种基础可以触发迁徙行为，很可能是由于基于其他昆虫类似行为可塑性的基因表达(利用遗传信息产生功能蛋白质的过程)的变化。导致这种现象的特定基因和生化过程还知之甚少。该项目将利用全球害虫棉铃虫研究迁徙的遗传基础，以应对环境线索，从而解决这一知识鸿沟。棉铃虫是一种夜蛾，存在于亚洲、非洲、中东和欧洲的大部分地区，能够侵染100多种寄主植物，包括许多农作物，毛虫阶段通过直接取食造成危害。正是棉铃虫在寄主作物之间迁徙数千公里的能力使其成为一种主要害虫。在实验室里描述任何生物体的迁徙并不容易。然而，通过使用Rothamsted Research设计和制造的计算机化系绳飞行系统，每晚最多可以飞行32个飞蛾，并以电子方式记录每一次飞行的距离、时间和速度。这使得对飞行性能的准确评估可以被用作迁徙行为与久坐行为的替代。直接从中国和希腊的寄主植物上收集的棉铃虫的飞行习性以前已经被描述过。使用一种名为rna-seq的技术(一种高通量测序工具，它可以确定每个样本中存在的RNA分子的完整序列和数量)，已识别出200多个在迁徙和久坐飞行表型之间存在差异表达的基因。这些基因中有许多在与迁徙生物学相关的过程中具有潜在的作用，包括飞行肌肉、新陈代谢和激素刺激。然而，目前尚不清楚哪些基因驱动兼性迁徙行为，或者它们的表达如何受到环境刺激的影响。这项提议的最初目标将是将棉铃虫幼虫暴露在三种环境线索(密度、光周期和营养)中的一种，这三种环境线索是夜蛾重要的迁徙触发因素。每个实验的成虫都将在飞行磨坊上飞行，以确定每个球杆对飞行能力的影响。最有希望的候选基因中的50个将被量化--从使用RNA-SEQ识别的200个基因--在单个蛾中，以确定哪些基因对每个线索的反应与飞行表现出最强的关联。RNA干扰(将双链RNA送入细胞以抑制基因表达)将被用来沉默根据其表达模式解释飞行倾向最大比例的五个基因，并检查它们在飞行工厂迁移中的作用。最后，同样的五个基因将在另一种生物--果蝇(Drophila Blackogaster)--中表达，看看它们是否会在为较小昆虫设计的一套改装飞行磨坊上提高飞行性能。这些技术的结合将有助于我们理解棉铃虫迁徙的遗传基础，特别是对环境线索的迁徙。这一结果将适用于其他重要的迁飞蛾，并将在可预见的未来为这一领域的研究设定基准。

Each year billions of insects migrate thousands of kilometres in search of food, shelter and places to breed. Embarking on such an arduous journey requires physiological, behavioural and morphological adaptations encoded by multiple genes. For many insects, migration is 'switched on' in anticipation of deteriorating habitats, allowing populations to succeed in the face of environmental stress. This ability to respond to environmental cues in a single generation requires a flexible genetic basis that triggers migratory behaviour and is highly likely to be due to changes in gene expression (the process of using genetic information to produce functional proteins) based on similar behavioural plasticity in other insects. The specific genes and biochemical processes which contribute to this phenomenon are poorly understood. This project will address this knowledge gap by studying the genetic basis of migration in response to environmental cues using the global insect pest, Helicoverpa armigera. H. armigera is a noctuid moth present throughout much of Asia, Africa, the Middle East and Europe and is capable of infesting over one hundred host plants including many crops, with the caterpillar stage causing damage through direct feeding. It is the ability of H. armigera to migrate thousands of kilometres between host crops that make it such a major pest. Characterising migration in the laboratory in any organism is not easy. However, by using a computerised tethered flight system, designed and built at Rothamsted Research, it is possible to fly up to 32 moths per night and electronically record the distance, time and speed of each individual flight. This permits an accurate assessment of flight performance which can be used as a proxy for migratory versus sedentary behaviour. The flight propensity of H. armigera collected directly from host plants in China and Greece has previously been characterised. Using a technology called RNA-seq (a high-throughput sequencing tool which determines both the full sequence and the number of RNA molecules present in each sample), over 200 genes have been identified that are differentially expressed between migratory and sedentary flight phenotypes. Many of these genes have potential roles in processes associated with migration biology including flight muscle, the metabolism and hormonal stimulation. It is not known, however, which genes drive facultative migratory behaviour or how their expression is affected by environmental stimuli. The initial objective of this proposal will be to expose larvae of H. armigera to one of three environmental cues (density, photoperiod and nutrition) that are important migratory triggers in noctuid moths. The adults from each of these experiments will be flown on the flight mills to determine the impact of each cue on flight ability. The expression of 50 of the most promising candidate genes will be quantified - from the 200 genes identified using RNA-seq - in individual moths to determine those genes that show the strongest association with flight in response to each cue. RNA interference (the delivery of double-stranded RNA into a cell to inhibit gene expression) will be used to silence the five genes that explain the greatest proportion in flight propensity according to their expression patterns and examine their role in migration on the flight mills. Finally, the same five genes will be expressed in a second organism, the fruitfly (Drosophila melanogaster), to see whether they induce increased flight performance on a modified set of flight mills designed for smaller insects. The combination of these techniques will contribute greatly to our understanding of the genetic basis of migration in H. armigera, and in particular, migration in response to environmental cues. The results will be applicable to other important migratory moths and will set the benchmark for this area of research for the foreseeable future.