Olfactory cues for host-seeking mosquitoes and insights into the evolution of odour coding

Abstract

Natural odours are complex blends of many components. Olfactory systems must be able to discriminate among these blends when they carry distinct ecological meaning. How do animals efficiently encode olfactory information? And how does this olfactory code evolve over time as animals adapt to new ecological niches and their sensory needs change? I shed light on these general questions by focusing on host-seeking behaviour in the yellow-fever mosquito Aedes aegypti. This mosquito exists in two forms: a generalist ancestral "forest" form and a derived "domestic" form that has evolved a strong preference for human odour within the last few thousand years. In Chapter 1, I quantitatively characterize the odour of a broad range of vertebrate hosts, focusing especially on identifying compounds that are enriched in human odour. I discuss how a subtle retuning of olfactory circuits to some of these compounds may have contributed to the evolution of domestic human-specialist mosquitoes from their generalist ancestors. These predictions are partly explored in a study in the Appendix, which found that those unique features of human odour are indeed used by domestic Ae. aegypti mosquitoes to target human hosts. In Chapter 2, I integrate evidence from a wide variety of fields to reveal that the most characteristic features of human odour identified in Chapter 1 originate from sebaceous skin oils. In Chapter 3, I explore the evolution of the atypical expression pattern of one Ae. aegypti olfactory receptor, laying a foundation for future work on how non-canonical receptor expression may influence olfactory coding. Altogether, this dissertation provides insight into which olfactory cues mosquitoes use to target hosts, how they may encode complex odour blends, and how this code may evolve.