Cellular dissection of the neural circuits processing and integrating skylight cues in the brain of the fruit fly Drosophila melanogaster

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The visual system must extract, process, and integrate information about different visual cues in order to inform a behavioral output. The dissection of neural circuits in the insect brain processing and integrating specific celestial cues like skylight polarization, colour gradients, and landmarks offers a unique opportunity to understand how processing and integration is achieved, from a cellular/synaptic to a physiological/behavioural level. The goal of this proposal is to provide a comprehensive functional characterization of the underlying circuit mechanisms in comparison with other sensory systems using Drosophila melanogaster as a model. Many insects use celestial cues to improve navigation over long distances, as well as more local orientation tasks. In the ‘compass pathway’, different skylight cues such as celestial polarization, colour and intensity gradients, or the position of a celestial body, are processed and conveyed via parallel pathways to the central brain. Electrophysiological recordings showed that integration of polarization and chromatic cues already begins in optic lobe cell types, becoming increasingly prominent in the ‘anterior optic tubercle’ (AOTU), an optic glomerulus in the central brain that serves a relay to the central complex. Importantly, fruit flies also use polarized light when orienting under the open sky, as well as under laboratory conditions. Like many other insects they harbor polarization-sensitive photoreceptors in a specialized ‘dorsal rim area’ of the adult eye, which manifest polarization-sensitive physiological responses. In preparation for this proposal we have accumulated preliminary data that motivate the experimental aims: Using molecular genetic tools we are now in the position to address important open questions about the functional connectivity of the neural circuits processing skylight polarization, as well as those circuit elements integrating polarization with other visual cues: (1) The activity of specific cell types will be visualized using genetically encoded indicators and their role for the behaving animal will be tested after inactivation. (2) Electron microscopic data spanning the entire fly brain (the ‘connectome’), will reveal both number and distribution of synaptic connections between cell types; (3) Functional synaptic connections will be validated by systematically silencing or activating cell types, while imaging the activity of downstream elements in the central brain; (4) The mechanistic role of specific neurotransmitters and their receptors will be elucidated using genetic mutants and cell type specific rescue. Despite studies on processing of skylight polarization, chromatic information, or small objects, the exact functional connectivity and integration within the compass pathway remain unknown. We will close this gap in knowledge, by dissecting the neural circuits connecting the eye to the AOTU, using a combination of anatomy, physiology, and behavior experiments.