[Cell lineage, determination and regulation in the female genital disc ofDrosophila melanogaster].

1. Under certain experimental conditions medially halved genital discs ofDrosophila melanogaster will form a complete, symmetrically duplicated genital apparatus. Female genital discs of heterozygous larvae were treated with X-rays at different developmental stages so that somatic crossingover (mitotic recombination) was induced which leads to genetically marked single cells. In the course of proliferation the homozygous cell will give rise to a clone which can be detected on the metamorphosed genital structures as a patch of marked bristles. This system was used to establish the cell lineagein situ and to investigate the mechanism of regulation in medially halved genital discs. 2. Marked clones on the genital apparatusin situ were restricted to either the vaginal or the anal plates, and to the left or to the right side of the symmetrical plates. Genital discs, irradiated between 3 and 60 h after egg laying gave rise to mosaic anal plates on which the marked clone frequently extended over the dorsal and ventral plates of one side. The results indicate that the female genital disc consists of two separate anlagen on each side as early as the time of blastoderm formation or shortly afterwards (3 h after egg laying): one anlage for the vaginal plate on each side, and a common anlage for the dorsal and ventral anal plates of each side. Dorsal and ventral anal plates share a common cell-pool at least up to the late second instar. 3. Medially halved genital discs in which marked clones had been induced were cultured in adult hosts and four days later transplanted back into larvae. Such fragments differentiated into symmetrical genital apparatuses which on the "new" side exhibited fewer but larger mosaics than on the "old" side. Clonally related bristles on the "new" side of the symmetrically duplicated genital apparatus occupied either only the vaginal or only the anal plates, and frequently extended over the dorsal and ventral halves of the anal plates. The results of the transplantation experiments indicate i) that the duplication of the genital structures is initiated from a limited zone of proliferation which forms separated blastemas for anal and vaginal plates, and ii) that the "new" halves of the dorsal and ventral anal plates arise from a common cell-pool. 4. In some developmental stages X-rays caused an enlargement of the anal plates and an increase in the bristle number on the anal and vaginal plates.