Cellular and synaptic adaptations of neural circuits processing skylight polarization in the fly

Abstract

1 8 Specialized ommatidia harboring polarization-sensitive photoreceptors exist in the 1 9 'dorsal rim area' (DRA) of virtually all insects. Although downstream elements have been 2 0 described both anatomically and physiologically throughout the optic lobes and the 2 1 central brain of different species, little is known about their cellular and synaptic 2 2 adaptations and how these shape their functional role in polarization vision. We have 2 3 previously shown that in the DRA of Drosophila melanogaster, two distinct types of 2 4 modality-specific 'distal medulla' cell types (Dm-DRA1 and Dm-DRA2) are post-synaptic 2 5 to long visual fiber photoreceptors R7 and R8, respectively. Here we describe additional 2 6 neuronal elements in the medulla neuropil that manifest modality-specific differences in 2 7 the DRA region, including DRA-specific neuronal morphology, as well as differences in 2 8 the structure of pre-or post-synaptic membranes. Furthermore, we show that certain cell 2 9 types (medulla tangential cells and octopaminergic neuromodulatory cells) specifically 3 0 avoid contacts with polarization-sensitive photoreceptors. Finally, while certain 3 1 transmedullary cells are specifically absent from DRA medulla columns, other subtypes 3 2show specific wiring differences while still connecting the DRA to the lobula complex, as 3 3 previously been described in larger insects. This hints towards a complex circuit 3 4 architecture with more than one pathway connecting polarization-sensitive DRA 3 5

Behavioral responses of free flyingDrosophila melanogasterto shiny, reflecting surfaces

“…The e-vectors of skylight scattered in the atmosphere create a polarization pattern across the sky, which changes according to the location of the sun (Wehner 2001, Heinze 2017, Mathejczyk and Wernet 2017). Many insects have evolved specialized visual systems, including specialized retinal detectors as well as underlying circuitry, to use this pattern for navigation and orientation (Labhart and Meyer 1999, Homberg 2015, Dacke and El Jundi 2018, Sancer, Kind et al 2019, Kind, Belušič et al 2020, Sancer, Kind et al 2020). Importantly, sunlight also becomes linearly polarized through reflections off shiny surfaces like water, animal or plant cuticles, or any nonmetallic shiny surfaces (Wehner, 2001).…”