The Drosophila visual system exhibits stereotyped connectivity and synaptic organization. a) Schematic of the Drosophila brain. Boxed region corresponds to the retina and optic lobe diagrammed in (a’). a’) Schematic of the 4 main optic lobe neuropils [figure panel modeled after; Nériec and Desplan (2016)]. R1–R6 photoreceptors innervate the lamina. R7 and R8 photoreceptor neurons innervate distinct layers in the medulla and form synaptic connections with different postsynaptic targets (not diagrammed). Boxed region in the medulla is enlarged in (b). b) Photoreceptor neurons exhibit class-specific synaptic numbers and organization [figure panel based on Berger-Müller et al. (2013) and Chen et al. (2014)]. R8 photoreceptors assemble approximately twice as many AZs in the medulla (∼50) as R7 photoreceptors (∼25). Moreover, synaptic organization is distinctly different in these 2 neuronal classes. In R8 photoreceptors, presynaptic AZs are distributed uniformly along the axonal terminal whereas, in R7 photoreceptors, they are concentrated near the distal tip of the axonal terminal. c) Synaptic addition in R7 photoreceptors takes place in a stepwise fashion, at bulbous filopodia (arrowhead), which are morphologically distinct from filamentous filopodia [figure panel based on Özel et al. (2019)]. In the current model, the cell surface receptor LAR initiates formation of bulbous filopodia, likely through local attachment at the presumptive synaptic site. Recruitment of the synaptic seeding factors, Liprin-α and Syd-1, stabilize the bulb while the RhoGEF Trio antagonizes formation of supernumerary bulbous filopodia. This pathway ensures that no more than 1–2 bulbous filopodia are formed throughout the synaptogenic period, limiting the number of partners that are competent to form a connection. Bulbous filopodia are long-lived (>8 min), but eventually retract. Following retraction, Brp is recruited to the nascent synapse where Liprin-α and Syd-1 reside.