The embryonic development of the Drosophila salivary glands provides several advantages for analysis of the genes involved in determination and differentiation. In the ectoderm of the germ band extended embryo, the glands begin development at a defined position under the control of several known pattern regulating genes. Differentiation then occurs within a few hours, implying that there are few steps leading from commitment to differentiation. Recent work has suggested a model for initial salivary placode determination that implicates both anterior-posterior and dorsal-ventral pattern forming genes in this initial step in salivary development. We not want to identify two kinds of genes that this model predicts may be involved in determination, and we want to begin examining the roles of additional genes in the later stages of salivary development. Three genes that affect dorsal-ventral patterning will be tested for positive effects on placode formation and a search will be started for genes that regulate the gene, fkh, that is expressed earliest in the developing placode. Since it is clear that cell-cell interactions are important in placode formation and further development, mutants lacking or having abnormal patterns of EGF homologues like Notch or Delta will be tested for effects on placode structure or on its subsequent invagination. Since the signal for placode invagination appears to originate from the posterior margin of the placode, segment polarity genes like wingless that are necessary for the posterior part of the parasegment will be tested for effects on invagination. Two genes, Toll and Dsrc28C, that may be involved in spreading this signal to the rest of the placode will be tested by expressing them ectopically throughout the placode. A method for marking particular cells in the developing embryo will be used to determine whether additional cells join the placode after it is formed and to locate the precursors for salivary duct cells. Formation of the ducts will also be examined in embryos mutant for Serrate or for genes of the spitz group, both of which may be involved in distinguishing between gland and duct cells.
