CELL LINEAGE, GROWTH, AND DETERMINATION IN IMAGINAL LEG DISCS OF DROSOPHILA MELANOGASTER

The pattern of cell lineage in the leg discs of Drosophila melanogaster was analyzed by means of X-ray-induced somatic crossing-over. The system is used to demonstrate that cell lineage data can yield important information regarding various developmental problems. Clonally related cells become arranged in longitudinal stripes on the leg, and the data indicate that part of the reason for this is that mitotic division products in the third larval instar become oriented in a radial fashion in the disc. The size of clones induced at specific times was used to estimate the cell number in the leg disc at these times. The presumptive leg disc appears to contain about 20 cells even in the 3-hour embryo, and to be mitotically inactive throughout the remainder of the embryonic period and the first half of the first larval instar. Thereafter it grows continuously with a doubling time of about 15 hours, until about the time of pupation. Observation of gynandromorphs, in which the male tissue was genetically marked, indicated also that the leg disc consists of about 20 cells when it is first delineated from the rest of the embryo. The results show that determination of the leg disc into regions does not occur until after the early third instar. This determinative event, as well as the initial determination of the leg disc, occurs in populations of cells rather than single cells. The results are incompatible with a previous theory of determination in imaginal discs. © 1969.