eagle is required for the specification of serotonin neurons and other neuroblast 7-3 progeny in the Drosophila CNS

During development of the Drosophila nerve cord, neuroblast 7-3 gives rise to a pair of mitotic sister serotonin neurons in each hemisegment. Here we show that the zinc finger gene eagle, which is expressed in neuroblast 7-3, is essential for specifying the fate of serotonin neurons. We find that loss-of-function eagle mutations produce an unusual differential phenotype with respect to the sister serotonin cells and that eagle is necessary for the maintenance of engrailed and zfh-2 expression in the serotonin neurons. We present a model that uniquely identifies all progeny neurons in the neuroblast 7-3 lineage based on the expression of specific molecular markers, position within the nerve cord and the effect of eagle loss-of-function mutations. Although serotonin is an important neurotransmitter conserved throughout the animal kingdom, we show that hypomorphic alleles of eagle can produce viable adults that have a dramatic reduction in the number of serotonin-producing neurons.