Cells in developing creatures must know their location so that they may differentiate apppropriately. In Drosophila imaginal discs this involves two processes: determination--decisions about which organ a cell will form; and specification (pattern formation)--decisions about which part of an organ a cell will form. There are two current models to account for specification. The clock model is based on data from regeneration experiments and suggests that cells must interact with their neighbors to regulate patterns. The compartment hypothesis derives from cell lineage studies and insists that during development cells suffer restrictions in the types of patterns they can form. The proposed research will investigate the relationship of the two models by studying cell lineage during regeneration. The disposition of genetically marked clones of cells will be followed in several different regenerating and mutant systems. Embryonic genital imaginal disc cells will be killed by radiation and the resulting patterns and cell lineage will be followed. Developmental genetics of the problem will be studied by investigating several pattern mutants including Abnormal Abdomen, apID, amx, and Frodo. To help understand compartmentalization, we will focus on the compartment border by studying twin spots and cell mixing along these boundaries with the scanning electron microscope. The two dorsal histoblast islands will be mapped using the blastoderm fate mapping technique to estimate the number of starts per cell. Determination will be studied by using mutants that change cell commitments. Aristapedia causes antennae to be replaced by legs at 17 degrees, but causes normal development at 29 degrees. Proboscipedia causes mouth parts to be replaced by antennae at 17 degrees, but by legs at 29 degrees. The phenotype of the double mutant will be studied to find the time that the two genes exert their effects on determination. Sex determination will be investigated by using somatic recombination to alter sexual genotypes during development. We will ask whether there comes a time in development after which a switch in sexual genotype does not result in a switch in sexual phenotype. The results of these experiments will be used to assess models trying to explain how cells define their position in a developing organism.