Understanding temperature adaptation in tropical insects

Understanding thermal adaptation is a priority if we wish to understand and mitigate the impacts of climate change. Organisms' current tolerances and their ability to adapt or move to new areas will determine whether they survive warming environments or not. There is currently a relative paucity of data on thermal adaptation in tropical as compared to temperate species. Tropical insects make up a large proportion of the Earth's biodiversity, but little is currently known about their ability to respond to climate change.

We propose to use a well-studied group of tropical ectotherms, the Heliconius butterflies, to assess thermal adaptation across altitudinal gradients, to determine the contributions of genetic and environmental variation to these traits, and to identify the underlying genetic loci. Within Heliconius there are several instances of subspecies that also show structuring by altitude. Using population genomic sequence data and the reference genome for H. melpomene we recently showed that three subspecies pairs of H. melpomene and H. erato across altitudinal gradients exhibit divergence in regions of the genome that contain candidate genes for thermal adaptation such as metabolic enzymes and heat shock proteins. We will characterise the ecophysiology of these species, through characterisation of the physiological differences found between populations and species. Within Ecuador these species have parallel altitudinal clines on the east and west side of the Andes allowing us to assess the replicability of any trends we identify.

Temperature can also be an important factor in delimiting the ecological niche and so in driving divergence and speciation. Research on the Heliconius system has largely focussed on the role of colour patterns in driving divergence between populations and species, but habitat differences are also present and thought to contribute to divergence. For example, the species pair H. melpomene and H. cydno on the western side of the Andes, which have become a classic system in speciation research, show divergent altitudinal ranges. The phenotypic differences driving divergence in habitat use have yet to be investigated, but physiological adaptations to temperature seem highly likely. Therefore, we will experimentally test for thermal adaptation differences between the sister clades H. melpomene and H. cydno/timareta, to assess the importance of temperature in determining altitudinal range and species distributions in these species.

We will use the extensive genomic resources available for Heliconius to perform genome scans and detailed association mapping analyses to identify loci responsible for thermal adaptation within H. melpomene. Identifying these loci will then allow us to address the question of whether genes involved in temperature adaptation show evidence for either introgression or divergent selection between species. Sharing of temperature adaptation genes between species could allow rapid adaptation to novel environments, while divergence would suggest thermal adaptation is important in maintaining species identities.

Research on Heliconius has flourished in recent years leading to many insights into the process of divergence and speciation in the genome. However, the ecological characters investigated have remained largely restricted to colour pattern. This project will be a major step towards establishing Heliconius as a more comprehensive model system for ecological genetics, making use of the existing knowledge, genomic resources and techniques available in this system to investigate broader ecological issues in the tropics.

1. Who will benefit from this research?

1.1 This research will benefit conservation policy makers and practitioners

1.2 This research will benefit those working in science education

1.3 This research will benefit wider society through increased engagement with science and conservation issues

2. How will they benefit from the research?

2.1 Losses of biodiversity due to habitat destruction and climate change are a major concern. Tropical insects make up a large and under-appreciated proportion of this diversity. Understanding adaptation to temperature and genetic structure in these species is necessary to predict how they may respond to climate change. This work will be among the first to directly address this. In particular in will help us understand the importance of altitudinal gradients in driving and maintaining genetic variation in temperature responses. Therefore results of this work may be useful for informing conservation policy and practice to conserve biodiversity, for example by providing information that can be taken into consideration when identifying priority tropical forest areas for conservation.

2.2 Temperature adaptation in butterflies is an excellent system for teaching the principles of evolution because of the appeal of butterflies and the general awareness of the issue of climate change. Our research will give a concrete example showing how environmental and genetic variation can lead to adaptive change. It will therefore provide an excellent evolutionary example for the classroom and one that would link across different areas of the curriculum.

2.3 Butterflies hold a unique place in the public imagination as evidenced by the coverage that previous Heliconius work has received in the popular press, and the enthusiastic responses we have received at outreach events. Butterfly research has been at the forefront of understanding animal responses to climate change and there is a general awareness of resulting changes in UK butterflies. This research would build on that interest and awareness, engaging the public in both the UK and Ecuador with climate and deforestation issues on a global scale and evolutionary biology more generally. The appeal of butterflies and tropical systems as well as the interest in climate change also means that this work has the potential to inspire young people to follow careers in science. This is of general benefit for innovation and society. Further, as the PI and visiting researcher are female scientists, this could be particularly beneficial in attracting women and girls into science.