ABSTRACT
Positional signals provided by immediate neighbours appear to direct developmental decisions in the eye of Drosophila. By a combined genetic and molecular approach the biochemical bases of the signal and reception mechanisms are being systematically dissected. Three key gene products have now been identified, sevenless is a transmembrane tyrosine kinase probably transducing positional signals that direct the R7 cell to its fate. The bride of sevenless gene product is on the signalling side of the mechanism and is required in R8 for R7 to develop. The type of protein bride of sevenless encodes is not yet known. The rough gene encodes a transcription factor on the signalling side required in R2 and RS for positional signals to be transmitted to neighbouring cells.
