DISSECTING THE TEMPORAL REQUIREMENTS FOR HOMEOTIC GENE-FUNCTION

Homeotic genes confer identity to the different segments of Drosophila . These genes are expressed in many cell types over long periods of ti me. To determine when the homeotic genes are required for specific dev elopmental events we have expressed the Ultrabithorax, abdominal-A and Abdominal-Bm proteins at different times during development using the GAL4 targeting technique. We find that early transient homeotic gene expression has no lasting effects on the differentiation of the larval epidermis, but it switches the fate of other cell types irreversibly (e.g. the spiracle primordia). We describe one cell type in the periph eral nervous system that makes sequential, independent responses to ho meotic gene expression. We also provide evidence that supports the hyp othesis of in vivo competition between the bithorax complex proteins f or the regulation of their down-stream targets.