Gradients of differentiation in wild-type and bithorax mutants of Drosophila.

We present evidence to show that differentiation in wing cells to produce hairs is synchronous over the distal 90% of the wing surface (approximately 28,000 cells). In spite of this synchrony within such a large area a temporal gradient exists between zones (in general anterior to posterior) on the animal surface with rather sharp boundaries in between. In order to evaluate the basis for the gradient we studied two mutants which carry different combinations of the genes of the bithorax complex. These were examined with respect to the temporal aspects of sensitivity to heat shock induction of the multihair phenocopy on wings and the time of initiation of the program of protein synthesis that is related to hair formation. Results show that the gradient observed is based on predetermined properties within specific areas of tissue rather than on the position of the cells in the animal.