Semaphorin-1a acts in concert with the cell adhesion molecules fasciclin II and connectin to regulate axon fasciculation in Drosophila

Semaphorins comprise a large family of phylogenetically conserved secreted and transmembrane glycoproteins, many of which have been implicated in repu lsive axon guidance events. The transmembrane semaphorin Sema-1a in Drosoph ila is expressed on motor axone and is required for the generation of neuro muscular connectivity. Sema-1a can function as an axonal repellent and medi ates motor axon defasciculation. Here, by manipulating the levels of Sema-1 a and the cell adhesion molecules fasciclin II (Fas II) and connectin (Conn ) on motor axons, we provide further evidence that Sema-1a mediates axonal defasciculation events by acting as an axonally localized repellent and tha t correct motor axon guidance results from a balance between attractive and repulsive guidance cues expressed on motor neurons.