GIFONDORWA, D.J.; LEISE, E.M.: Does the Apical Ganglion Undergo a Form of Programmed Cell Death during Metamorphosis in Ilyanassa obsoleta

The apical ganglion (AG), also known as the apical sensory organ, is an unpaired ganglion that contains approximately 25 neurons in competent larvae of the marine gastropod Ilyanassa obsoleta. This ganglion is anterior to the dorsal commissure that connects the cerebral ganglia and it innervates the velum, the larval swimming and feeding organ. Recent evidence suggests that the AG is a site for the reception of metamorphic cues in a related mollusc (Hadfield, et al., Biol. Bull. 2000, 198:67-76). In I. obsoleta, by 4 days after metamorphic induction, the AG disappears through an as yet, unidentified mechanism. The AG displays nitric oxide synthase (NOS)-like immunoreactivity in most of its neuronal somata (Thavaradhara and Leise, J. Neurocytol., submitted) and previous work in our laboratory has demonstrated that inhibition of NOS induces competent larvae to metamorphose. Because nitric oxide (NO) can prevent cells from undergoing apoptosis, a form of programmed cell death (PCD), we hypothesize that inhibition of NOS induces metamorphosis by initiating loss of the AG. Alternatively, the AG could be lost by having its neurons merge with the subjacent cerebral ganglia. We are testing our hypothesis by applying the TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) assay to sections of the AG taken from larvae that have been induced to metamorphose with a NOS inhibitor. The TUNEL assay labels DNA that has been fragmented during the PCD program. Currently, we have detected cell fragments in the region of the AG by 84 hr after metamorphosis and are examining younger juveniles.