Genetic analysis of a drosophila neural cell adhesion molecule: Interaction of fasciclin I and abelson tyrosine kinase mutations

Drosophila fasciclin I is a homophilic cell adhesion molecule expressed in the developing embryo on the surface of a subset of fasciculating IONS axons, all PNS axons, and some nonneuronal cells. We have identified protein-null mutations in the fasciclin I (fas I) gene, and show that these mutants are viable and do not display gross defects in nervous system morphogenesis. The Drosophila Abelson (abo proto-oncogene homolog encodes a cytoplasmic tyrosine kinase that is expressed during embryogenesis primarily in developing CNS axons; abl mutants show no gross defects in CNS morphogenesis. However, embryos doubly mutant for fas 1 and abl display major defects in CNS axon pathways, particularly in the commissural tracts where expression of these two proteins normally overlaps. The double mutant shows a clear defect in growth cone guidance; for example, the RP1 growth cone (normally fas 1 positive) does not follow its normal path across the commissure.