Cell surface control of the layer specific targeting in the Drosophila visual system

To achieve the precise wiring of axons in the brain required to form a fine architecture, a molecular level interaction between axons and their targets is necessary. The Drosophila visual system has a layered and columnar structure which is often found in the brain of vertebrates. With powerful genetic tools for its analysis, the Drosophila visual system provides a useful framework to examine the molecular mechanisms of axon targeting specificity. The medulla is the second optic ganglion in the Drosophila optic lobe, and is subdivided into ten layers. Among the eight photoreceptor types, R7 and R8 pass through the first optic ganglion lamina and innervate the medulla. In the medulla, R7 and R8 axons grow in a distinct manner to reach their final target layers: M6 and M3, respectively. The axons from R7 and R8 take characteristic steps to extend toward their target layer. In this review, we discuss the formation of the Drosophila optic lobe and the molecular mechanisms of layer specific targeting of R8 axons in the medulla. Fundamental and comprehensive understanding of the crosstalk of growing axons and target regions in the Drosophila optic lobe will elucidate the general principles applicable to more complex nervous systems.