A LATE ROLE FOR A SUBSET OF NEUROGENIC GENES TO LIMIT SENSORY PRECURSOR RECRUITMENTS IN DROSOPHILA EMBRYOS

In Drosophila, mutations in a class of genes, the neurogenic genes, pr oduce an excess of neurons. This neural hyperplasia has been attribute d to the formation of more than the normal number of neuronal precurso r cells at the expense of epidermal cells. In order to find out whethe r the neurogenic genes only act at this intial step of neurogenesis, w e studied the replication pattern of the sensory organ precursor cells by monitoring BrdU incorporation in embryos mutant for Notch (N), Del ta (Dl), mastermind (mam), almondex (amx), neuralized (neu), big brain (bib) and the Enhancer of split-Complex (E(spl)-C). Using temperature sensitive alleles of two of the neurogenic genes, DI and N, we also i nduced an acute increase of replicating sensory precursors by shifting briefly to the restricted temperature. We have found that the loss of function of all the seven neurogenic loci that were tested causes an increase in replicating sensory precursor cells, consistent with the m odel that these neurogenic genes normally participate in the process o f restricting the number of neuronal precursors. Whereas the temporal pattern of replication appeared normal in mutants of five of the seven neurogenic loci, in N and mam embryos replicating PNS cells are prese nt beyond the time when they normally undergo replication. Experiments with colchicine suggest that many of these late replicating cells may be newly emerging precursors and probably not additional cell divisio ns of already recruited precursors. Thus, different neurogenic genes m ay be required over different periods of time for the specification of sensory precursor cells.