Cell death influences structural reorganization of the larval nervous system during metamorphosis. Soumya Banerjee, Matthew Siefert, Marcus Toral, Joyce Fernands. Zoology, Miami Univ, Oxford, OH.

   In keeping with the shift of locomotor control to the thorax, the thoracic ganglion expands during metamorphosis, while the abdominal ganglion is much smaller in proportion. Another significant structural change occurs in the posterior abdominal nerves, which can be visualized as the formation of the terminal nerve trunk. In the larva, eight abdominal nerves directly exit the abdominal ganglion; whereas in the adult, the first three pairs of nerves, A1-A3 exit directly, and nerves A4-A8 are bundled into a terminal nerve trunk (TNT). Examining the abdominal segmental nerve fusion during early metamorphosis indicated that the larval to adult transition becomes evident at the onset of the second day of metamorphosis. To investigate the role of cell death on the restructuring of ganglion we examined reaper null mutants (H99/XR38). Instead of three pairs of abdominal nerves, four or five pairs of nerves exit directly from the abdominal ganglion (80%; n=25), indicating a defect in segmental nerve fusion. In addition, instead of one terminal nerve trunk, two terminal nerve trunks are observed in reaper null adults (90%; n=25). Examining the peripheral innervation indicated that significantly more (p<0.005; n=10) secondary axonal branches and boutons are present in reaper null mutant flies as compared with controls. Studying the fate of dHb9 expressing motor neurons in wild type flies indicated that almost 30% of these neurons undergo programmed cell death during the larval to adult transition. Currently, we are following the fate of this subset of neurons in reaper null mutant flies during this period of development.