Alterations in gene dosage cause serious perturbations in the developmental program of organisms, yet very little is known about the molecular genetic basis of gene dosage effects. Our laboratory studies the Triplo-lethal locus of Drosophila (Tpl)as a model for understanding gene dosage effects. Tpl is the only genetic locus known that is lethal when present in three copies. It is also haplo-lethal. Animals with either three copies or one copy of Tpl die as late embryos or early first instar larvae, with no obvious gross morphological defects. Tpl is a complex locus, and may consist of redundant information, since point mutations that eliminate its function have never been obtained. Now that cloning of the Tpl region is nearly complete, rearrangement breakpoints will be located on the map, transcribed regions will be identified, and a candidate gene or genes for Tpl will be chosen for further analysis. These candidates will then be studied by germ-line and somatic transformation experiments as an assay for Tpl function. The developmental phenotype of embryos who are dying because of altered Tpl dosage, will also be examined in detail, with particular emphasis on the pattern of cell death which occurs. Since gross developmental anomalies are not seen, it seems probable that Tpl is involved in some fundamental cellular function, and the pattern of cell death which occurs will give us insights into the nature of the defect. Mitotic recombination will also be used to generate somatic clones of cells carrying altered Tpl dosage, which will reveal whether the effects of Tpl are cell autonomous, and if there are developmental stages, or tissues that are not sensitive to Tpl aneuploidy. There are also mutations in a nearby locus, Su(Tpl), which efficiently suppress the triplo-lethal but not the haplo-lethal phenotype of Tpl. Using these mutations in Su(Tpl), and revertants that we will generate, we will clone the locus, and study its interaction with Tpl.