THE DROSOPHILA BEATEN PATH GENE ENCODES A NOVEL SECRETED PROTEIN THATREGULATES DEFASCICULATION AT MOTOR AXON CHOICE POINTS

In the Drosophila embryo, at specific choice points along the major mo tor nerves, subsets of motor axons defasciculate and then steer into t heir muscle target regions. Here we describe the analysis of beaten pa th (beat), a gene required for the selective defasciculation of motor axons at these choice points. In beat mutant embryos, motor axons fail to defasciculate and bypass their targets. This phenotype is suppress ed by mutations in FasII and conn, two genes encoding cell adhesion mo lecules expressed on motor axons, suggesting that beat provides an ant iadhesive function. beat encodes a novel secreted protein that is expr essed by motoneurons during outgrowth. Rescue and ectopic expression e xperiments support the model that Beat protein is secreted by motor ax ons where it functions to regulate their selective defasciculation at specific choice points.