The nervous system must develop more intricate functional connections between its cells that any of the other systems of the body. To understand mechanisms providing such specificity, model subsystems of the brain often are used. The fruit fly, Drosophila, has large compound eyes that provide an excellent model for development because classical and molecular genetic analysis can be used, and certain cell types are easy to identify. Within this compound eyes are repeating subunits containing eight photoreceptor cells, labelled R1 to R8. The first one to appear differentiated during normal development is R8, and it is thought to play a critical role in the subsequent specification of the remaining 7 photoreceptor cells. A mutant has been found that produces a retina aberrant in pattern, so named "rap" gene, including cloning and sequencing. Results will clarify how inductive interactions can regulate cellular specificity, and will have impact on developmental biology as well as neuroscience and visual science.
