Metamorphosis of tangential visual system neurons in Drosophila

To learn about construction of the adult nervous system, we studied the dif ferentiation of imaginal neurons in the Drosophila visual system. OL2-A and OL3 are tangential neurons that display dFMRFa neuropeptide gene expressio n in adults but not in larvae. The two large OL2-A neurons are generated ne ar the end of the embryonic period and already show morphological different iation at the start of metamorphosis. The numerous small OL3 neurons are ge nerated postembryonically and first detected later in metamorphosis. The on set of dFMRFa transcription coincides with that of neuropeptide accumulatio n in OL2-A neurons, but it precedes peptide accumulation in the OL3 neurons by days. Altering each of the five conserved sequences within the minimal 256-bp OL dFMRFa enhancer affected in vivo OL transcriptional activity in t wo cases: alteration of a TAAT element greatly diminished and alteration of a 9-bp tandem repeat completely abolished OL2-A/OL3 reporter activity. A 4 6-bp concatamer containing the TAAT element, tested separately, was not act ive in OL neurons. We propose a model of neuronal differentiation at metamo rphosis that features developmental differences between classes of imaginal neurons. (C) 2000 Academic Press.