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The establishment of dorsal-ventral asymmetry of the Drosophila embryo requires a group of genes that act maternally. None of the previously identified dorsal-ventral axis genes are known to produce asymmetrically localized gene products during oogenesis. We show that rhomboid (rho), a novel member of this group, encodes a protein that is localized on the apical surface of the dorsal-anterior follicle cells surrounding the oocyte. Loss of rho function causes ventralization of the eggshell and the embryo, whereas ectopic expression leads to dorsalization of both structures. Thus, spatially restricted rho is necessary and sufficient for dorsal-ventral axis formation. We propose, based on these observations and double mutant experiments, that the spatially restricted rho protein leads to selective activation of the epidermal growth factor receptor in the dorsal follicle cells and subsequently the specification of the dorsal follicle cells.

The Drosophila peripheral nervous system comprises four major types of sensory element: external sense organs (such as mechano-sensory bristles), chordotonal organs (internal stretch receptors), multiple dendritic neurons, and photoreceptors. During development, the selection of neural precursors for external sense organs requires the proneural genes of the achaete-scute complex, which encode basic-helix-loop-helix transcription factors. These genes do not, however, control precursor selection for chordotonal organs or photoreceptors, raising the question of whether other proneural genes exist or a different mechanism of neurogenesis operates. Here we show that atonal (ato), originally isolated as a proneural gene for chordotonal organs, is also the proneural gene for photoreceptors. Pattern formation in the Drosophila eye involves a succession of cell fate specifications. Of the eight photoreceptors within each ommatidium of the compound eye, the photoreceptor R8 is the first to appear in the eye imaginal disc, right behind the morphogenetic furrow. The appearance of other photoreceptors (R1-7) follows in a defined sequence that is thought to arise by induction from R8 (refs 8, 9, 11, 12). We find that photoreceptor formation requires the function of atonal at the morphogenetic furrow and that atonal is specifically required for R8 selection. Formation of other photoreceptors does not directly require atonal function, but does depend on R8 selection by atonal. Thus, photoreceptors are selected by two mechanisms: R8 by a proneural mechanism, and R1-7 by local recruitment.

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.

A P-element vector has been constructed and used to generate lines of flies with single autosomal P-element insertions. The lines were analyzed in two ways: (1) the identification of cis-acting patterning information within the Drosophila genome, as revealed by a lacZ reporter gene within the P element, and (2) the isolation of lethal mutations. We examined 3768 independent lines for the expression of lacZ in embryos and looked among these lines for lethal mutations affecting embryonic neurogenesis. This type of screen appears to be an effective way to find new loci that may play a role in the development of the Drosophila nervous system.

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.