The Conserved Ig Superfamily Member Turtle Mediates Axonal Tiling in Drosophila

Restriction of adjacent same-type axons/dendrites to separate single columns for specific neuronal connections is commonly observed in vertebrates and invertebrates, and is necessary for proper processing of sensory information. Columnar restriction is conceptually similar to tiling, a phenomenon referring to the avoidance of neurites from adjacent same-type neurons. The molecular mechanism underlying the establishment of columnar restriction or axonal/dendritic tiling remains largely undefined. Here, we identify Turtle (Tutl), a member of the conserved Tutl/Dasm1/IgSF9 subfamily of the Ig superfamily, as a key player in regulating the tiling pattern of R7 photoreceptor terminals in Drosophila. Tutl functions to prevent fusion between two adjacent R7 terminals, and acts in parallel to the Activin pathway. Tutl mediates homophilic cell–cell interactions. We propose that extrinsic terminal–terminal recognition mediated by Tutl, acts in concert with intrinsic Activin-dependent control of terminal growth, to restrict the connection made by each R7 axon to a single column.