The mode of action of “discless” mutations in Drosophila melanogaster

Abstract

The development of five different “discless” lethal mutants of Drosophila melanogaster was studied by using whole mounts and histological procedures as well as by radiation-induced mitotic recombination. Rudimentary imaginal discs were found in both newly hatched first-instar and late-third-instar larvae homozygous for each of the mutations. Rather than interfering with the establishment of imaginal discs, the mutations were found to interfere with cell proliferation not only in the imaginal discs but also in the larval central nervous system, the imaginal rings of the salivary gland and foregut, and the gonads. Production of blood cells by the lymph gland was severely reduced in all of the mutants. Autonomous effects on the growth of homozygous clones in a heterozygous background were demonstrated for four of the five mutations. Although some of the mutations had, in general, a more extreme effect on cell proliferation than did others, the effects on different organ systems were not clearly correlated. The mutations also caused a slight reduction in the final cell size in the nonproliferating larval epidermis and histoblasts, but only the most extreme mutation studied caused a reduction in cell number in these tissues. It is concluded that these mutations are not imaginal disc specific, but that they interfere with functions required for cell proliferation in general. Several possible explanations for their lack of effects on embryonic cell proliferation are discussed.