Identification of novel regulators of apoptosis during metamorphosis. Gina Castelvecchi¹, Yunsik Kang¹, Anne Sapiro², Sarah Ives¹, Arash Bashirullah¹. 1) Division of Pharmaceutical Sciences, University of Wisconsin-Madison, Madison, Wisconsin, United States of America; 2) Department of Genetics, Stanford University, Stanford, California, United States of America.

   The transformation from larvae to pupae during metamorphosis heavily relies on the massive and rapid destruction of obsolete larval tissues. This process is triggered, in part, by the stage- and tissue-specific activation of apoptosis in response to expression of IAP antagonists reaper (rpr) and head involution defective (hid). Animals carrying loss-of-function mutations in critical regulators of apoptosis like the initiator caspase Nedd2-like caspase (Nc), the effector caspase Drosophila ICE (drice) and the apaf-1-related killer (Ark) die during metamorphosis, presumably as a result of defects in remodeling the future adult. To identify novel regulators of apoptosis, we conducted a large-scale EMS mutagenesis screen. First, we generated over 8,600 new lethal mutations on the third chromosome and selected those that died exclusively during metamorphosis. We then conducted dominant GMR-rpr and hs-rpr modifier screens and identified 17 complementation groups among the ~900 newly identified metamorphosis-specific lethals. We identified loss-of-function alleles of Nc and a gain-of-function allele of diap-1, validating the efficacy of the screen for identifying regulators of apoptosis. The strongest and most frequently hit complementation group maps to a novel and evolutionarily conserved gene we named bulsa (immortal in Korean). Mutations in bulsa block all endogenous programmed cell death during metamorphosis while the overexpression of bulsa is sufficient to trigger caspase activation, demonstrating that bulsa is a critical regulator of apoptosis. We will present our initial characterization of bulsa and our progress in identification of the remaining loci identified in our screen.