SALIVARY DUCT DETERMINATION IN DROSOPHILA - ROLES OF THE EGF RECEPTORSIGNALING PATHWAY AND THE TRANSCRIPTION FACTORS FORK HEAD AND TRACHEALESS

Organogenesis in Drosophila embryos begins at 4-5 hours of development as the expression of organ-specific genes is initiated. The salivary primordium, which occupies the ventral epidermis of parasegment 2, is among the earliest to be defined. It is soon divided into two distinct regions: the more dorsal pregland cells and the more ventral preduct cells. We show that it is the opposing activities of the Drosophila EG F receptor (DER) Signaling pathway and the Fork head transcription fac tor that distinguish these cell types and set up the boundary between them. DER signaling acts ventrally to block fork head expression in th e preduct cells, thereby restricting gland identity to the more dorsal cells. Fork head in turn blocks expression of duct-specific genes in the pregland cells, thereby restricting duct identity to the more vent ral cells. A third regulatory activity, the Trachealess transcription factor, is also required to establish the identity of the preduct cell s, but we show that it acts independently or downstream from the DER:f ork head confrontation. In trachealess mutants, subdivision of the sal ivary primordium occurs normally and the dorsal cells form glands, but the ventral cells are undetermined. We present a model proposing that trachealess is the crucial duct-specific gene that Fork head represse s to distinguish pregland from preduct cells.