Biophysical and functional consequences of cuticular hydrocarbon variation in ants

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Cuticular hydrocarbons (CHCs) cover the body of nearly all insects. Their composition is highly variable: intraspecific (e.g. acclimatory) variation concerns quantitative changes of the same hydrocarbons, whereas different species can possess entirely different hydrocarbons. The CHC layer protects against desiccation, against pathogens, and serves as communication signal. While the role of CHC variation for communication is being intensively studied, considerably less is known about how it affects waterproofing and the ability to fend off pathogens. Both functions are largely determined by interactions between hydrocarbon molecules, which affect phase behaviour, melting behaviour, and viscosity of the CHC layer. We still hardly know how CHC variation determines these physical traits, and how they affect CHC functionality. Here, I propose to study how CHC variation affects physical CHC traits and the ability to waterproof and protect against fungal pathogens, in ants. Firstly, I will study intraspecific variation in two ant species: the extent of acclimatory CHC changes (the reaction norm) among continental and maritime populations. During acclimation, insects change their CHC profiles to enhance drought resistance. Being exposed to stronger fluctuations, continental populations should have evolved a larger reaction norm and thus a better ability to cope with fluctuations, than maritime ones. Colonies of both species will be collected along a maritime-continental gradient across Europe and acclimated at different temperature regimes. Subsequently, we will investigate CHC changes and acclimation-related changes in biophysical properties, drought survival, water permeability and CHC-induced growth inhibition of fungal pathogens. They will be compared across regimes, but also across populations. Secondly I will study interspecific variation, focusing on the most prominent CHC difference among insect species: whether they are dominated by unsaturated hydrocarbons or by multiply branched alkanes. This distinction explains a major part of interspecific CHC variation, whereas nearly all insects have n-alkanes. We will compare species of the two profile types concerning physical traits, waterproofing ability and the inhibition of fungal growth. In particular, we will investigate how these traits depend on the current temperature and how they change along with acclimatory CHC changes. Taken together, this project will link chemical composition to the physical phenotype and functionality of CHC layers. Especially the link between CHC composition (beyond n-alkanes) and physical traits is novel and has hardly been studied before. It will reveal which chemical and physical traits are under biological selection, and hence contribute to our understanding of CHC evolution. Moreover, we will be able to assess how species and conspecific populations can deal with different climates. This will be crucial to predict how they respond to climate change.