Cell lineage and development in the larval epidermis ofDrosophila melanogaster

Larval and adult gynandromorphs were produced to study the origin and development of the larval epidermis inDrosophila. In mosaic larvae the genetic markermal was used to score the genotype of all the cells in the epidermis and thus to follow the gynandromorphic border through this large continuous sheet of cells. This mosaic border proved to be rather convoluted, such that even small areas of 46 cells were mosaic in 23% of the cases. From the similarity of this value to the frequency of mosaicism in the adult legs, it was concluded that the epidermis of the larval segments T₁-A₇ (thoracic-abdominal) arises from about 2000 blastoderm cells, thus requiring two rounds of cell division to achieve the 7500 cells present at final differentiation of the larval epidermis. An increased frequency of mosaic borders was observed at the ventral midline of the larvae, indicating the "loss" of 16% of the cells in the circumference of the blastoderm which invaginate ventrally during gastrulation. A smaller increase in frequency of borders (4.8%) was found on the dorsal midline, reflecting the formation of the extraembryonic membranes. Finally, an increase in frequency of mosaic borders was observed on all segment boundaries, which can be interpreted as the result of cell lineage restrictions between the larval epidermal cells of adjacent segments arising at the blastoderm stage. In all mosaic larvae, the genotype of the six imaginal histoblast clusters per segment closely corresponded to the genotype of the surrounding larval cells. The imaginal histoblasts thus originate as clusters of cells within the large continuous sheet of cells giving rise to the larval epidermis.