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Neurogenesis in Drosophila begins with the formation of neuronal precursors, which give rise to neurons of individual identity. To find out whether there are genes that are expressed in most or all neuronal precursors and are involved in controlling particular aspects of neuronal differentiation, we used the enhancer-trap method to screen for such "neuronal precursor genes." One gene of this group is prospero . Our mutant analysis indicates that prospero regulates other neuronal precursor genes and is essential for the axonal outgrowth and pathfinding of numerous central and peripheral neurons. prospero encodes a large nuclear protein with multiple homopolymeric amino acid stretches and is expressed in neuronal precursors early during their formation. It is probably generally required for proper neuronal differentiation.

The daughterless (da) gene is known to have separate maternal and zygotic functions: Maternally supplied daughterless activity is required for proper sex determination and dosage compensation in female embryos, whereas loss of zygotically supplied da+ activity causes embryonic lethality in both male and female embryos. We have found that the zygotic da+ activity is necessary for neural development: The use of neuron-specific antibodies and beta-galactosidase-marked X chromosomes has revealed that in both male and female embryos deletions or strong mutations of the da gene remove all peripheral neurons and associated sensory structures without disrupting the epithelium from which they derive. Partial da+ function causes partial removal of peripheral neurons. Our results indicate that da+ is required for the formation of peripheral neurons and their associated sensory structures.