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. Author manuscript; available in PMC: 2014 Dec 1.
Published in final edited form as: Curr Opin Neurobiol. 2013 Aug 6;23(6):10.1016/j.conb.2013.07.004. doi: 10.1016/j.conb.2013.07.004

Figure 2.

Figure 2

Synaptic specificity in Drosophila. (a) Neuronal projection in the Drosophila medulla. Laminae in the medulla are labeled M1–M6; M1 is the shallowest layer while M6 is the deepest. Additional deeper layers do not receive input from photoreceptor cells or laminar neurons and are not shown in the diagram. R7 (orange) and R8 (red) photoreceptors project to the M6 and the M3/M4 layers of the medulla, respectively. Lamina neurons L1–L5 (green) also project to specific layers in the medulla (indicated by labels M1–M6 in the diagram). (b) Cadherins and Plexin-Semaphorin signaling control L3 lamina neuron targeting. CadN asterisks interactions between L3 and additional laminar neurons L1 and L5 direct L3 to a temporary layer, while Sema1a (yellow triangle) and PlexA (orange bar) prevent targeting to deeper layers. Subsequently, PlexA (in orange, produced by tangential fibers labeled in blue) interacts with Sema-1a (from L3 neurons) to direct targeting to the final M3 layer via inhibitory signaling. CadN is not involved in the final targeting to the M3 layer, so L1/L5 neurons are not shown in later stages of the targeting process. (c) R8 photoreceptor targeting is dependent on interactions between two factors: the atypical cadherin flamingo (grey T) and Golden Goal (black L). Golden Goal first directs the growing R8 axon to guide to the temporary M1 layer. After the R8 axon reaches the temporary M1 layer, Flamingo antagonizes Golden Goal activity to promote R8 targeting to the deeper M3 target layer. Interactions between Flamingo and Golden Goal (expressed in R8 axons) and Flamingo (expressed on target cells in the M3 layer) then direct R8 axons to stop extending once they have reached the M3 layer. Netrin signaling is also involved in R8 targeting, as secreted Netrin (blue V) is localized in the M3 layer where it acts as an attractant for R8 axons expressing the Netrin receptor Frazzled (brown bar). Loss of the Netrin signal prevents R8 targeting to the M3 layer and instead results in premature R8 axon terminations in the M1 or M2 layers.