CHROMOPHORE-ASSISTED LASER INACTIVATION OF PATCHED PROTEIN SWITCHES CELL FATE IN THE LARVAL VISUAL-SYSTEM OF DROSOPHILA

The Drosophila segment-polarity gene patched (pk) is an integral compo nent of the segmentation gene cascade acting in the early embryo. At l ater stages of embryogenesis, ptc is expressed in the primordia of epi thelial placodes of a specific portion of the brain, the optic lobes. Mutant analysis shows that the lack of ptc activity alters the fate of optic-lobe primordia precursors. In ptc mutants they give rise to sup ernumerary neurons in the larval light-sensory system, termed Bolwig o rgan, which is derived from precursor cells next to the optic-lobe anl agen. We specifically eliminated ptc protein by chromophore-assisted l aser inactivation (CALI) in late wild-type embryos. Such embryos show a normal segment pattern, but they develop phenocopies equivalent to t he phenotype of ptc mutant Bolwig organs. Our results demonstrate that the CALI technique can be applied to separate genetic functions at di fferent developmental stages of a living organism and that the segment -polarity gene ptc is redeployed to functionally discriminate between distinct developmental pathways in adjacent pools of precursor cells.