Pattern formation in imaginal discs of Drosophila melanogaster after irradiation of embryos and young larvae

Embryos and first instar larvae of Drosophila melanogaster were X-irradiated in order to study pattern formation in discs damaged at early stages. After treatment, appendages were found in which some pattern elements were duplicated and others were absent. In some strains legs were preferentially duplicated, and in others, antennae were preferentially duplicated. Duplicated appendages were mirror images and resulted most frequently when animals were irradiated during late embryonic or early larval stages. Appendages varied from those which showed complete duplication of only the distal parts (claws or aristae), to those which showed duplication of almost the entire appendage. Examination of the cuticular patterns in duplicated mesothoracic legs showed that in legs with complete duplication solely in distal regions, only extreme lateral leg parts were duplicated, and medial leg parts were absent. In legs with duplication extending into proximal regions, much of the lateral side was duplicated, and only extreme medial parts were missing. The situation for partially duplicated antennae was similar. Prothoracic legs were found fused in some X-rayed flies. The cuticular patterns were almost perfect mirror images, although the amount of fusion varied widely between different individuals. Apparently the pattern forming processes in the right and left first leg discs are coordinated in X-rayed animals. The results were consistent with a model embracing a gradient of developmental capacity in the early disc similar to that postulated to exist in the late third instar leg disc. This model is also consistent with results of various surgical experiments reported in the literature. Several predictions of the model are explained including the possible mode of action of a specific class of mutants which may affect pattern formation by altering a morphogenetic gradient.