Axon guidance by growth-rate modulation.

Guidance of axons by molecular gradients is crucial for wiring up the developing nervous system. It often is assumed that the unique signature of such guidance is immediate and biased turning of the axon tip toward or away from the gradient. However, here we show that such turning is not required for guidance. Rather, by a combination of experimental and computational analyses, we demonstrate that growth-rate modulation is an alternative mechanism for guidance. Furthermore we show that, although both mechanisms may operate simultaneously, biased turning dominates in steep gradients, whereas growth-rate modulation may dominate in shallow gradients. These results suggest that biased axon turning is not the only method by which guidance can occur.