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GRAVITAXIC BEHAVIOR IN DROSOPHILA MELANOGASTER.   J.D. Armstrong, M.J. Texada, E.L. Carter, E.S. Kuo, C.M. Nadorff and K.M. Beckingham. Dept. of Biochem. and Cell Biol., Rice Univ., Houston TX.

The fruitfly Drosophila melanogaster is an excellent model for genetic analysis of behavior. We are applying both traditional genetic and transgenic approaches to the study of gravitational responses as a route to identifying i) the molecular signaling pathways, ii) sense organs, and iii) the neuronal circuitry involved in gravity responses. Gravitaxic behavior in Drosophila is assayed using a vertical maze system with eight up/down choice points and nine final exit routes. Flies progress through the maze and distribution of a given mutant population across the exits is compared  to controls. Supplementary tests eliminate mutants with problems in locomotion, anatomy, or general responsiveness. Two types of mutagen have been employed:  EMS (chemically induced point mutations) and P{GAL4} (transposon insertion mutations). 750 mutant strains have been tested. Several mutant strains with maze exit positions i) much higher or ii) much lower than controls have been isolated. For some, the genes affected have been identified. These include an evolutionarily conserved PDZ-domain containing protein with a known cell signaling role downstream of the initial visual response. The known role of this protein in mediating fast cell signaling in vision is pleasingly suggestive of a similar fast sensory mechanism for gravity. Also identified are genes for several transcription factors, notably one involved in the development of the nervous system. A complementary targeted approach is also being pursued. A brain region termed the central complex (CC), interconnected to most, if not all, sensory modalities, has been directly linked to a range of behaviors in insects. We have studied 30 mutations that affect the neuronal circuitry of the CC and demonstrated a clear link between this neuronal structure and gravitaxic behavior. In summary, our studies in Drosophila have identified genes with potential roles in both the signaling pathways and neuronal circuitry required for gravitaxic responses and have identified at least one brain region involved in coordinating gravitaxic behavior. Further, they suggest that the mechanisms underlying gravitaxic responses are conserved across the animal kingdom. (Supported by a NASA Specialized Center for Research and Training [NSCORT] Grant at Rice University NAG5-4072.)