Genetic analysis of growth cone guidance in Drosophila: fasciclin II functions as a neuronal recognition molecule.

fasiclin II (fas II), a member of the immunoglobulin superfamily, was previously characterized and cloned in grasshopper. To analyze the function of this molecule, we cloned the Drosophila fas II homolog and generated mutants in the gene. In both grasshopper and Drosophila, fasciclin II is expressed on the MP1 fascicle and a subset of other axon pathways. In fas II mutant Drosophila embryos, the CNS displays no gross phenotype, but the MP1 fascicle fails to develop. The MP1, dMP2, and vMP2 growth cones fail to recognize one another or other axons that normally join the MP1 pathway. During their normal period of axon out-growth, these growth cones stall and do not join any other neighboring pathway. Thus, fasciclin II functions as a neuronal recognition molecule for the MP1 axon pathway. These studies serve as molecular confirmation for the existence of functional labels on specific axon pathways in the developing nervous system.