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ABSTRACT Synapses are often precisely organized on dendritic arbors, yet the role of synaptic topography in dendritic integration remains poorly understood. Utilizing electron microscopy (EM) connectomics we investigate synaptic topography inDrosophila melanogasterlooming circuits, focusing on retinotopically tuned visual projection neurons (VPNs) that synapse onto descending neurons (DNs). Synapses of a given VPN type project to non-overlapping regions on DN dendrites. Within these spatially constrained clusters, synapses are not retinotopically organized, but instead adopt near random distributions. To investigate how this organization strategy impacts DN integration, we developed multicompartment models of DNs fitted to experimental data and using precise EM morphologies and synapse locations. We find that DN dendrite morphologies normalize EPSP amplitudes of individual synaptic inputs and that near random distributions of synapses ensure linear encoding of synapse numbers from individual VPNs. These findings illuminate how synaptic topography influences dendritic integration and suggest that linear encoding of synapse numbers may be a default strategy established through connectivity and passive neuron properties, upon which active properties and plasticity can then tune as needed. 

SUMMARY The brain exhibits remarkable neuronal diversity which is critical for its functional integrity. From the sheer number of cell types emerging from extensive transcriptional, morphological, and connectome datasets, the question arises of how the brain is capable of generating so many unique identities. ‘Terminal selectors’ are transcription factors hypothesized to determine the final identity characteristics in post-mitotic cells. Which transcription factors function as terminal selectors and the level of control they exert over different terminal characteristics are not well defined. Here, we establish a novel role for the transcription factorbroadas a terminal selector inDrosophila melanogaster. We capitalize on existing large sequencing and connectomics datasets and employ a comprehensive characterization of terminal characteristics including Perturb-seq and whole-cell electrophysiology. We find a single isoformbroad-z4serves as the switch between the identity of two visual projection neurons LPLC1 and LPLC2.Broad-z4is natively expressed in LPLC1, and is capable of transforming the transcriptome, morphology, and functional connectivity of LPLC2 cells into LPLC1 cells when perturbed. Our comprehensive work establishes a single isoform as the smallest unit underlying an identity switch, which may serve as a conserved strategy replicated across developmental programs. 

Behaviorally relevant, higher order representations of an animal’s environment are built from the convergence of visual features encoded in the early stages of visual processing. Although developmental mechanisms that generate feature encoding channels in early visual circuits have been uncovered, relatively little is known about the mechanisms that direct feature convergence to enable appropriate integration into downstream circuits. Here we explore the development of a collision detection sensorimotor circuit in Drosophila melanogaster, the convergence of visual projection neurons (VPNs) onto the dendrites of a large descending neuron, the giant fiber (GF). We find VPNs encoding different visual features establish their respective territories on GF dendrites through sequential axon arrival during development. Physical occupancy, but not developmental activity, is important to maintain territories. Ablation of one VPN results in the expansion of remaining VPN territories and functional compensation that enables the GF to retain responses to ethologically relevant visual stimuli. GF developmental activity, observed using a pupal electrophysiology preparation, appears after VPN territories are established, and likely contributes to later stages of synapse assembly and refinement. Our data highlight temporal mechanisms for visual feature convergence and promote the GF circuit and the Drosophila optic glomeruli, where VPN to GF connectivity resides, as a powerful developmental model for investigating complex wiring programs and developmental plasticity.