Understanding the genetic mechanisms of phenotypic plasticity in insect migration

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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.