Clonal analysis of primordial disc cells in the early embryo of Drosophila melanogaster

Single cells were marked by X-ray-induced somatic recombination at the blastoderm stage (3 hr) and at 7 and 10 hr of development, when the embryos are relatively insensitive to radiation damage. The resulting clones of genetically marked cells were analyzed on the adult cuticle. For the three stages examined there is a decrease in clone size with developmental age indicating proliferation of the primordial disc cells which give rise to the adult cuticular structures. The extent of the clones has been used to study the specificity of determination in the primordial disc cells at the time of induction. A significant fraction of the clones induced at all three stages extend from the antenna into the head. In the thorax, some of the clones induced at 3 hr extend from the second leg into the wing, thus indicating that these cells are not yet disc-specifically determined. No clones, however, were found extending from the first leg into the second or from the second into the third, and by 7 hr all thoracic clones were restricted to a single disc. The results provide evidence that determination at the blastoderm stage is not yet disc-specific.