Abstract
In most animals, the brain controls the body via a set of descending neurons (DNs) that traverse the neck. DN activity activates, maintains or modulates locomotion and other behaviors. Individual DNs have been well-studied in species from insects to primates, but little is known about overall connectivity patterns across the DN population. We systematically investigated DN anatomy in Drosophila melanogaster and created over 100 transgenic lines targeting individual cell types. We identified roughly half of all Drosophila DNs and comprehensively map connectivity between sensory and motor neuropils in the brain and nerve cord, respectively. We find the nerve cord is a layered system of neuropils reflecting the fly's capability for two largely independent means of locomotion -- walking and flight -- using distinct sets of appendages. Our results reveal the basic functional map of descending pathways in flies and provide tools for systematic interrogation of neural circuits.
Highlights
The evolution of nervous systems is dominated by the process of cephalization, in which anterior ganglia fuse to create a brain that integrates information from a number of specialized sensory organs (Bullock and Horridge, 1965)
We observed a similar pattern of labeling in four animals and estimated the total number of descending neurons (DNs) at ~350 on each side of the brain (~700 total) based on the maximum cell body count among preparations
We screened over two thousand activation domain (AD)/DNA-binding domain (DBD) combinations for expression restricted to single DN cell types
Summary
The evolution of nervous systems is dominated by the process of cephalization, in which anterior ganglia fuse to create a brain that integrates information from a number of specialized sensory organs (Bullock and Horridge, 1965) In most animals, this large cephalic structure communicates with motor centers via a diverse population of descending neurons (DNs), with axons that run in connectives, or tracts, to more posterior ganglia. Recent combinatorial genetic techniques (Luan et al, 2006) make it possible to target individual neurons in the fly nervous system for visualization and manipulation (Aso et al, 2014; Wolff et al, 2015; Wu et al, 2016) We applied these techniques to identify individual DNs in Drosophila and create a large collection of selective driver lines that will facilitate their future study. This organization likely reflects both the function of each pathway’s member cells and the evolutionary history of winged insects
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