Roberts, Ruairí Joshua Van; (2024) The evolution of olfactory circuits in drosophilid larvae. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This project conducts a comparison between the larvae of two closely related species of Drosophilid: Drosophila erecta and Drosophila melanogaster. Past literature has de- scribed these species as occupying very different ecological niches. Additionally, data gathered by the host lab shows that larvae of these species display different olfactory preferences. This establishes the question: where in these animals’ neural circuitry may we find differences that explain these behavioural differences? This project primarily focuses on the olfactory circuitry using a comparative connec- tomics approach. This entails the manual reconstruction of the larval D. erecta olfac- tory circuit neuron by neuron, the annotation of synaptic connections between these neurons, and the subsequent analysis of these circuits. This project finds that while the olfactory circuitry is highly conserved in terms of its constituent neurons there is a wealth of circuit differences which may, in part, explain some of these species’ ob- served behavioural differences. This analysis is based both on neuronal morphology and connectivity and focuses on the olfactory sensory neurons, local neurons that pro- cesses this olfactory information, and olfactory relay neurons that send this information deeper into the brain. In addition, this project analyses optogenetic behavioural experiments designed around findings from the comparative connectomics. Local neurons that appear to have different tuning across the two species were both activated and inactivated and changes in the larval olfactory behaviour observed and compared. This relied on many larvae being tracked simultaneously as they were either attracted or repelled by an odorant. It appears that perturbing these local neurons can indeed, partially, induce one species’ behaviour to resemble that of the other species. The wider implications of both the connectomics and behavioural studies is to address how different animal species can evolve different behaviours and what evolutionary strategies might be common in closely related animals.

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