Drosophila Dscam is required for divergent segregation of sister branches and suppresses ectopic bifurcation of axons.

Axon bifurcation results in the formation of sister branches, and divergent segregation of the sister branches is essential for efficient innervation of multiple targets. From a genetic mosaic screen, we find that a lethal mutation in the Drosophila Down syndrome cell adhesion molecule (Dscam) specifically perturbs segregation of axonal branches in the mushroom bodies. Single axon analysis further reveals that Dscam mutant axons generate additional branches, which randomly segregate among the available targets. Moreover, when only one target remains, branching is suppressed in wild-type axons while Dscam mutant axons still form multiple branches at the original bifurcation point. Taken together, we conclude that Dscam controls axon branching and guidance such that a neuron can innervate multiple targets with minimal branching.