Understanding the genetic mechanisms of phenotypic plasticity in insect migration

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.

Migration is an essential life-history strategy for several of the world's most damaging agricultural insect pests. For many insects, migration occurs in anticipation of deteriorating habitats allowing populations to succeed in the face of spatial and temporal environmental variability. This facultative behaviour, expressed in a single generation, requires phenotypic plasticity in response to environmental stimuli that is likely to be driven by changes in gene expression. The specific genes involved, however, remain largely unknown. The proposal will fill this knowledge gap in one of the most important and invasive crop pests, the cotton bollworm moth (Helicoverpa armigera). By combining state-of-the-art phenotype characterisation (tethered flight) with forefront genomic and post-genomic technologies the overall aim of the project is to determine the relative contribution of a set of migratory candidate genes - previously identified using RNA-seq - on adult flight activity in response to key sensory cues for insect migration experienced during the larval stage. Three environmental factors (density, photoperiod and nutrition) will be manipulated in the laboratory and the effect on flight phenotypes determined. The contribution of a core set of 50 candidate genes on flight performance will be deciphered using quantitative PCR (qPCR) to measure gene expression in insects from a single family line of H. armigera exposed to each environmental treatment. The five genes that explain the greatest proportion of phenotypic variance in flight will be 'silenced' using RNA interference (RNAi) and the impact of gene knockdown on flight assessed. Finally, functional validation of each gene will be performed by transgenic over-expression in Drosophila and the effect on flight performance assessed using a modified set of flight mills. The results generated from this project will be applicable to other noctuid moth pests and contribute to predictive models for pest forecasting.

The scientific output from this proposal will have economic and societal benefits for the agricultural sector, the crop protection industry, academic stakeholders, collaborating institutes and the wider public. The rationale of the proposal is designed to fill a clear gap in our knowledge - the genetic control of migration in insect pests - using one of the most important pests of global agriculture Helicoverpa armigera as a model species. In the first instance, the project will benefit the agricultural sector using an understanding of migratory genotypes to improve the capacity of integrated pest management programmes to predict and model future outbreaks of invading migratory pests. The research offers a novel strategy for controlling damaging pests and contributing towards sustainable and improved crop yields in the future. Although the proposal will work exclusively with H. armigera, the benefits have both a national and international perspective, as many noctuid moths (including H. armigera) are important or potential immigrant pests into the UK, and the findings from this work will be relevant to other species within this group. The dispersal ability of insect pests has a significant consequence for the spread of insecticide resistance genes (including resistance to Bt cotton in the case of H. armigera) and this research will have clear benefits to academic groups and the crop protection industry trying to limit the evolution of resistance. Results from this project will be disseminated through peer-reviewed publications (at least three open access papers by December 2018), two international conferences (September 2016 & November 2018) and one national conference (Summer 2018) and visits to collaborating institutes (supported by the project). This proposal is at the forefront of a new research field, that of 'migratory genomics', and will be of considerable interest to an international migration research community. The findings will be circulated as widely as possible and facilitated by Dr Chapman's (leader of the Insect Migration and Spatial Ecology Group) extensive network in this area. The project will develop a wide range of molecular techniques which will provide novel resources for H. armigera research including qPCR assays, RNAi and transgenic expression of H. armigera genes in Drosophila.

The research will also appeal to entomology and Lepidopteran enthusiasts and appropriate platforms will be used to share the outcomes and ideas from this work. I aim to target this audience at the annual Royal Entomological Society conference in the summer of 2018. There is a great deal of public interest in animal migration. I will therefore conduct at least one public engagement activity (Summer 2016) to highlight this area as a fascinating part of natural history that is, in some cases (e.g. the monarch butterfly) currently under threat due to climate change. The research also provides an ideal means to inspire young scientists about an exciting area of biology and I will conduct one schools activity (by December 2017) which will hopefully encourage school children to think about a career in biology. Finally, I will write one 'popular science' article (e.g. The Biologist) (by December 2018) to further disseminate the work to a wider non-specialist audience.