Neuronal development in the Drosophila compound eye: photoreceptor cells R1, R6, and R7 fail to differentiate in the retina aberrant in pattern (rap) mutant.

The compound eye of Drosophila is a reiterated pattern of 800 unit eyes known as ommatidia. In each ommatidium there are eight photoreceptor neurons (R1-R8) and an invariant number of accessory cells organized in a precise manner. In the developing eye, specification of cell fates is triggered by sequential inductive events mediated by cell-cell interactions. The R8 photoreceptor neuron is the first cell to differentiate and is thought to play a central role in the recruitment of the remaining photoreceptor cells. Our previous work demonstrated that mutations in the retina aberrant in pattern (rap) locus lead to abnormal pattern formation in the compound eye. Genetic mosaic experiments demonstrated that for normal retinal patterning to occur, rap gene function is required only in the photoreceptor cell R8. In this study we analyzed the R cell composition of developing as well as the adult eyes of rap mutants employing a variety of R cell specific markers. We show that in rap mutants, although some of the R8-specific markers show normal expression patterns, other aspects of the R8 cell differentiation are abnormal. In addition, the cells R1, R6, and R7 fail to differentiate properly in rap mutants. These results suggest that the rap gene encodes an R8-specific function that plays a role in the determination of the photoreceptor cells R1, R6, and R7.

A product of the prune locus of Drosophila is similar to mammalian GTPase-activating protein.

The X-linked prune (pn) eye-colour mutation of Drosophila melanogaster has a highly specific, complementary lethal interaction with the conditional dominant Killer of prune (awdK-pn) mutation. Although awdK-pn flies have no apparent phenotype on their own, pn awdK-pn double mutants die as second or third larval instars. The awd locus encodes a nucleoside diphosphate kinase, an enzyme that catalyses the transfer of high-energy phosphate bonds between nucleoside diphosphates and nucleoside triphosphates, which is essential for the normal development of Drosophila. Analysis of the pn locus has suggested that the complementary DNA, TcD37, encodes a putative pn+ product. Here we report the nucleotide sequence of TcD37 and the similarity of its deduced protein product to the catalytic domain of mammalian GTPase-activating proteins (GAPs); GAPs stimulate the GTPase activity of Ras (ref. 6), which are plasma membrane-bound proteins involved in the regulation of cell proliferation and differentiation. These results suggest that the Drosophila TcD37 protein participates in a biochemical pathway similar to that of Ras and GAPs in mammals and yeast. We propose that the interaction between pn and awd is due to a neomorphic mutation that enhances the ability of AwdK-pn nucleoside diphosphate kinase to induce a regulatory GTPase into a GTP-bound 'on' state, whereas Pn modulates the activity of this GTPase either by switching it to a GDP-bound 'off' state or by interfering with its effector function.

Fused rhabdomeres (fur) in Drosophila: an eye mutation that alters rhabdomere morphology and retinal function.

In the Drosophila compound eye, the photoreceptor cells are organized in highly precise units, the ommatidia. In each photoreceptor cell, the primary photopigment, opsin, is contained in the rhabdomere, an ordered array of densely packed microvilli. A genetic and phenotypic analysis of a new X-linked. P element-induced mutation, fur, (fused rhabdomeres) is presented. Light and electron microscope studies show that mutations at the fur locus result in the fusion of the adjacent rhabdomeres in the developing eye and the fusion takes place during the pupal stage of eye development. Electrophysiological experiments indicate that the fur mutant photoreceptors have reduced sensitivity to light and lack a PDA (prolonged depolarizing afterpotential), a response characteristic of normal photoreceptor cells. Recombination and deficiency mapping localize fur to the proximal region of the X chromosome. Reversion analysis indicates the fur mutant is the result of a P element insertion. These studies suggest that the fur locus encodes a gene that has specific roles in rhabdomere morphogenesis and retinal function.

From monoclonal antibody to gene for a neuron-specific glycoprotein in Drosophila.

A monoclonal antibody (MAb24B10), derived from mice immunized with Drosophila retina, exclusively stains photoreceptor cells in the retina and their axonal projections to the optic ganglia. The antigen (Ag24B10) is a 160-kDa glycoprotein comprising about 0.8% of the retina protein. By microsequencing, 19 of the first 21 amino acids at the NH2-terminal end of the protein have been determined. Using synthetic oligonucleotide probes corresponding to a portion of this amino acid sequence, we isolated a homologous lambda genomic clone. A partial DNA sequence of this clone, along with blot experiments on genomic DNA and RNA, indicate that this clone is part of the structural gene for Ag24B10. By in situ hybridization, the gene was localized to the tip of chromosome 3R.

Neuronal development in the Drosophila retina: monoclonal antibodies as molecular probes.

The compound eye of D. melanogaster is a reiterative pattern of facets, each containing eight photoreceptor cells in a precise arrangement. This pattern is established in the eye imaginal disc during the third larval instar. A wave of morphogenesis sweeps from posterior to anterior across the disc, leaving in its wake organized clusters of photoreceptor cells. We have used monoclonal antibodies to highlight pattern elements that are not readily observable by other techniques. Monoclonal antibodies can be used to identify the molecules associated with particular patterns, providing links between observable structures and the genes. As an example, we present the purification and N-terminal sequence of a glycoprotein antigen specific to photoreceptor cells and their axons.

Isolation and characterization of membranes from Drosophila melanogaster.

As a prerequisite for examining the membranes of neurological mutants, we have undertaken to fractionate and characterize membranes derived from heads of adult and whole larvae of wild type Canton-S (C-S) Drosophila. Of particular interest to us are membrane fractions rich in putative brain membrane marker enzyme acetylcholinesterase.