The zebrafish diwanka gene controls an early step of motor growth cone migration

During vertebrate embryogenesis different classes of motor axons exit the s pinal cord and migrate on common axonal paths into the periphery. Surprisin gly little is known about how this initial migration of spinal motor axons is controlled by external cues. Here, we show that the diwanka gene is requ ired for growth cone migration of three identified subtypes of zebrafish pr imary motoneurons. In diwanka mutant embryos, motor growth cone migration w ithin the spinal cord is unaffected but it is strongly impaired as motor ax ons enter their common path to the somites, Chimera analysis shows that diw anka gene activity is required in a small set of myotomal cells, called ada xial cells. We identified a subset of the adaxial cells to be sufficient to rescue the diwanka motor axon defect. Moreover, we show that this subset o f adaxial cells delineates the common axonal path prior to axonogenesis, an d we show that interactions between these adaxial cells and motor growth co nes are likely to be transient. The studies demonstrate that a distinct pop ulation of myotomal cells plays a pivotal role in the early migration of ze brafish motor axons and identify the diwanka gene as a somite-derived cue r equired to establish an axonal path from the spinal cord to the somites.