DROSOPHILA EYE DEVELOPMENT - NOTCH AND DELTA AMPLIFY A NEUROGENIC PATTERN CONFERRED ON THE MORPHOGENETIC FURROW BY SCABROUS

Loss of function mutations of scabrous and conditional alleles of Notc h and Delta affect the pattern of morphogenetic furrow development. By studying differentiation of R8 cells, the first photoreceptor neuron subtype to differentiate, we show that all furrow cells pass through a n R8-competent stage. Function of Notch and scabrous is necessary if m ost of these cells are to attain other cell fates. The scabrous gene c onfers a regular pattern on the morphogenetic furrow, restricting R8 d ifferentiation to alternating groups of cells. Notch and Delta functio n to restrict the R8 fate to a single cell in each group. Without scab rous gene function, action of Notch and Delta on the entire morphogene tic furrow results in a disorganised pattern of ommatidia arising from a disorganised array of single R8 cells. Aspects of the scabrous muta nt phenotype also suggest a secondary role in selecting a single R8 ce ll from competent clusters. We show that scabrous expression preceeds changes in the apical profiles of morphogenetic furrow cells that iden tify ommatidial precursor cells, and also preceeds changes in levels o f Notch and Delta expression. The pattern of initiation of sea express ion depends on sea gene function, indicating that patterning of the mo rphogenetic furrow depends on the pattern of posterior columns. Our re sults suggest that in the eye, Notch and Delta amplify and refine a mo rphogenetic landscape generated by scabrous. Cell determination in oth er tissues and organisms might also be molded in a two-step process wh ere initial inhomogeneities determined by one protein provide a contex t for subsequent development.