Fasciclin II controls proneural gene expression in Drosophila.

Fasciclin II (Fas II), an NCAM-like cell adhesion molecule in Drosophila, is expressed on a subset of embryonic axons and controls selective axon fasiculation. Fas II is also expressed in imaginal discs. Here we use genetic analysis to show that Fas II is required for the control of proneural gene expression. Clusters of cells in the eye-antennal imaginal disc express the achaete proneural gene and give rise to mechanosensory neurons; other clusters of cells express the atonal gene and give rise to ocellar photoreceptor neurons. In fasII loss-of-function mutants, the expression of both proneural genes is absent in certain locations, and, as a result, the corresponding sensory precursors fail to develop. In fasII gain-of-function conditions, extra sensory structures arise from this same region of the imaginal disc. Mutations in the Abelson tyrosine kinase gene show dominant interactions with fasII mutations, suggesting that Abl and Fas II function in a signaling pathway that controls proneural gene expression.