SPITZ, A DROSOPHILA HOMOLOG OF TRANSFORMING GROWTH-FACTOR-ALPHA, IS REQUIRED IN THE FOUNDING PHOTORECEPTOR CELLS OF THE COMPOUND EYE FACETS

Cell type specification and differentiation in the developing Drosophi la compound eye begins in the morphogenetic furrow. In the furrow, cel ls are organized into evenly spaced preclusters and there is a synchro nized arrest of the cells' mitotic cycle in G1. We report that recessi ve spilt loss-of-function mutations affect compound eye development. S pitz is homologous to the human transforming growth factor-alpha. In m osaic clones, spilt function is required in the first photoreceptor ce lls to differentiate for normal ommatidial development spilt loss-of-f unction mutations are dominant suppressors of Egfr(E) gain-of-function mutations of the epidermal growth factor-receptor gene. These data su ggest that the spilt product is a precluster promoting factor. spilt t ranscription increases abruptly in the morphogenetic furrow, the obver se of Egfr expression. We present a model for the expression of, and c ellular requirement for, this growth factor homolog.