CLONAL ANALYSIS OF PRIMORDIAL DISK CELLS IN EARLY EMBRYO OF DROSOPHILA-MELANOGASTER

Single cells were marked by X-ray-induced somatic recombination at the blastoderm stage (3 h) and at 7 and 10 h of development, when the embryos were relatively insensitive to radiation damage. The resulting clones of genetically marked cells were analyzed on the adult cuticle. For the 3 stages examined clone size decreased with developmental age indicating proliferation of the primordial disk cells which give rise to the adult cuticular structures. The extent of the clones was used to study the specificity of determination in the primordial disk cells at the time of induction. A significant fraction of the clones induced at all 3 stages extended from the antenna into the head. In the thorax some clones induced at 3 h extended from the 2nd leg into the wing, indicating that these cells were not yet disk-specifically determined. No clones extended from the 1st leg into the 2nd or from the 2nd into the 3rd, and by 7 h all thoracic clones were restricted to a single disk. The results provide evidence that determination at the blastoderm stage is not yet disk-specific.