Notch is required for successive cell decisions in the developing Drosophila retina

Ligand-independent activation of the sevenless receptor tyrosine kinase changes the fate of cells in the developing Drosophila eye

Cell fate in the developing eye is determined by a cascade of inductive interactions. In this process, the sevenless protein--a receptor tyrosine kinase--is required for the specification of the R7 photoreceptor cell fate. We have constructed a gain-of-function sevenless mutation (SevS11) by overexpressing a truncated sevenless protein in the cells where sevenless is normally expressed. In SevS11 mutant flies, all sevenless-expressing cells initiate neural development. This results in the formation of multiple R7-like photoreceptors per ommatidium. Therefore, sevenless activity appears to be necessary and sufficient for the determination of R7 cell fate. These results illustrate the central role receptor tyrosine kinases can play in the specification of cell fate during development.

Developmental analysis of the facets, a group of intronic mutations at the Notch locus of Drosophila melanogaster that affect postembryonic development

The activity of the Notch locus of Drosophila melanogaster during embryogenesis is necessary for the correct segregation of neural from epidermal lineages. The action of Notch is not confined to embryogenesis but is also essential for normal development during the postembryonic stages. Its action is pleiotropic, as revealed by the existence of several classes of mutations which affect various imaginal structures. Here, we examine a group of six recessive mutations, the facets (fa, fa3, fag, fag-2, fafx and fasw), which affect eye and optic lobe morphology and have been previously shown to be associated with the insertion of transposable elements into an intronic region of Notch. Using both somatic recombination and gynandromorph analysis, we find that their behavior in a mosaic analysis is not identical. While in the majority of alleles abnormal Notch function in the retina is sufficient to induce optic lobe abnormalities, in the case of fag-2, a considerable number of individuals having mosaic retinas exhibit normal optic lobe structure. All the facet alleles appear to behave in a cell-autonomous manner. A developmental analysis of the eye and optic lobe defects associated with the facet mutations support the contention that Notch may be involved not only in the formation of certain structures but also in their maintenance.

Spacing differentiation in the developing Drosophila eye: a fibrinogen-related lateral inhibitor encoded by scabrous

In the development of multicellular organisms a diversity of cell types differentiate at specific positions. Spacing patterns, in which an array of two or more cell types forms from a uniform field of cells, are a common feature of development. Identical precursor cells may adopt different fates because of competition and inhibition between them. Such a pattern in the developing Drosophila eye is the evenly spaced array of R8 cells, around which other cell types are subsequently recruited. Genetic studies suggest that the scabrous mutation disrupts a signal produced by R8 cells that inhibits other cells from also becoming R8 cells. The scabrous locus was cloned, and it appears to encode a secreted protein partly related to the beta and gamma chains of fibrinogen. It is proposed that the sca locus encodes a lateral inhibitor of R8 differentiation. The roles of the Drosophila EGF-receptor homologue (DER) and Notch genes in this process were also investigated.

Genes required for specifying cell fates in Drosophila embryonic sensory nervous system

The nervous system contains a diverse group of cells. Specification of the correct cell fate is obviously important for the proper function of the nervous system, yet how are the fates of different neurons determined during development? Very little is known about the underlying molecular mechanisms used in the mammalian nervous system. How, for example, are certain cells directed to form pyramidal cells rather than local interneurons? In the fruit fly Drosophila melanogaster, and the nematode Caenorhabditis elegans, some progress has been made in studying neuronal fate determination. For instance, in Drosophila, a number of genes acting at different levels have been found to control this process. They function to (1) endow a subset of the ectodermal cells in the early embryo with the potential to become neuronal precursors, (2) commit some of these cells to the fate of neuronal precursors, (3) specify the identity of these neuronal precursors, and (4) specify the identity of the individual progeny cells of a neuronal precursor. In this review, we discuss first the rationale of the genetic approach, then outline the working hypothesis and, finally, briefly describe the genes known to be involved in the formation of the sensory nervous system in Drosophila. We also discuss the prospects for extrapolating these molecular mechanisms and principles to vertebrate and invertebrate neural development.

seven in absentia, a gene required for specification of R7 cell fate in the Drosophila eye

Signals from directly adjacent cells are critical for the development of the R7 photoreceptor cell in the Drosophila eye. We have identified a gene, seven in absentia (sina), for which loss of function results in the R7 precursor cell adopting the fate of a nonneuronal cell type. Function of the sina gene is required only in R7 for correct R7 cell development. The sina transcription unit is located within the intron of a gene that encodes an R7 cell-specific opsin. The sina protein, which has a potential metal binding domain, is localized in the nuclei of several ommatidial precursor cells including R7, and sina expression in R7 appears before R7 overtly begins to differentiate. These data indicate that the sina gene product is necessary at a stage in the determination of R7 cell fate when R7 receives and interprets developmental signals from neighboring cells, and possibly acts by regulating gene expression.

A dual function of the Notch gene in Drosophila sensillum development

We have investigated the function of the neurogenic gene Notch (N) during development of the adult sensilla of Drosophila. Heat pulses were applied to flies carrying the temperature-sensitive Notch allele Nts1 at different larval and pupal stages. We can show that the reduction of Notch+ function during a short interval prior to the onset of sensillum precursor division, resulting from a heat pulse between 0 and 14 hr after puparium formation (apf), leads to an increase in microchaete precursors at the expense of epidermal cells. The structure and cellular composition of the sensilla produced by these supernumerary precursors are normal. Later heat pulses which include the interval immediately after sensillum precursor division (14-20 hr apf) lead, among the progeny of the sensillum precursors, to a hyperplasia of sensory neurons, at the expense of accessory cells. The resulting "sensilla" consist of neurons only and lack the external cuticular structures (i.e., shaft, socket). These results demonstrate that similar mechanisms both of which involve the function of the Notch gene may be operating to sort out (premitotic) sensillum precursors from epidermal precursors and (postmitotic) sensory neurons from accessory cells. They further show that in postmitotic sensillum cells the differentiative fate is not yet irreversibly fixed, but presumably requires cell-cell interaction to become established.

Isolation and expression of scabrous, a gene regulating neurogenesis in Drosophila

Mutations in the Drosophila scabrous (sca) gene affect eye and bristle development, leading to irregular spacing of ommatidia and bristle duplications in the adult fly. We have cloned the sca gene by P-element tagging. The sca transcription unit is 12 kb and consists of four exons that are joined in a 3.2-kb mRNA. In an enhancer trap screen we have isolated several P[lacZ] insertions close to the sca transcription start site. We have examined the expression pattern of sca by in situ hybridization to sca transcripts, by beta-galactosidase localization in the P[lacZ] lines, and by immunocytochemistry with an anti-sca antiserum. During embryogenesis, sca is expressed in a dynamic pattern associated with neural development. During imaginal development, sca is mainly expressed in the R8 photoreceptor precursor cells in the eye imaginal disc and in sensory organ precursor cells in other discs. In the wing disc, sca expression is coextensive with the anlagen for bristles and is controlled by genes of the achaete-scute complex. Based on its loss-of-function phenotype, expression pattern, and the predicted structure of its product, a secreted peptide with homology to the fibrinogen gene family, we propose that sca encodes a signal involved in lateral inhibition within individual domains of the developing nervous system.

The single-minded gene of Drosophila is required for the expression of genes important for the development of CNS midline cells

The single-minded (sim) gene of Drosophila encodes a nuclear protein that plays a critical role in the development of the neurons, glia, and other nonneuronal cells that lie along the midline of the embryonic CNS. Using distinct cell fate markers, we observe that in sim mutant embryos the midline cells fail to differentiate properly into their mature CNS cell types and do not take their appropriate positions within the developing CNS. We further present evidence that sim is required for midline expression of a group of genes including slit, Toll, rhomboid, engrailed, and a gene at 91F; that the sim mutant CNS defect may be largely due to loss of midline slit expression; and that the snail gene is required to repress sim and other midline genes in the presumptive mesoderm.

The Drosophila neurogenic locus mastermind encodes a nuclear protein unusually rich in amino acid homopolymers

The neurogenic loci of Drosophila are required for proper partitioning of ectodermal cells into epidermal versus neural lineages. The loci appear to encode components of a developmental pathway involving cellular communication. In an effort to understand the role of the neurogenic locus mastermind in these processes, we have characterized its expression and sequence. The locus produces a number of transcripts that accumulate ubiquitously during early embryogenesis but more specifically in the central nervous system during later stages. Sequence analysis of a major cDNA product predicts an unusual protein containing an abundance of amino acid homopolymers and charge clusters typical of regulatory molecules. Nearly half of the mass of the predicted protein derives from only three amino acids: glutamine, glycine, and asparagine. Immunohistochemical studies of the protein in cell culture and early embryos show that the protein accumulates predominantly in the nucleus.

Xotch, the Xenopus homolog of Drosophila notch

During the development of a vertebrate embryo, cell fate is determined by inductive signals passing between neighboring tissues. Such determinative interactions have been difficult to characterize fully without knowledge of the molecular mechanisms involved. Mutations of Drosophila and the nematode Caenorhabditis elegans have been isolated that define a family of related gene products involved in similar types of cellular inductions. One of these genes, the Notch gene from Drosophila, is involved with cell fate choices in the neurogenic region of the blastoderm, in the developing nervous system, and in the eye-antennal imaginal disc. Complementary DNA clones were isolated from Xenopus embryos with Notch DNA in order to investigate whether cell-cell interactions in vertebrate embryos also depend on Notch-like molecules. This approach identified a Xenopus molecule, Xotch, which is remarkably similar to Drosophila Notch in both structure and developmental expression.

The homeo domain protein rough is expressed in a subset of cells in the developing Drosophila eye where it can specify photoreceptor cell subtype

The Drosophila homeo box gene rough is required in photoreceptor cells R2 and R5 for normal eye development. We show here that rough protein expression is limited to a subset of cells in the developing retina where it is transiently expressed for 30-60 hr. The rough protein is first expressed broadly in the morphogenetic furrow but is rapidly restricted to the R2, R3, R4, and R5 precursor cells. Ubiquitous expression of rough under the control of the hsp70 promoter in third-instar larvae suppresses the initial steps of ommatidial assembly. Structures derived from other imaginal discs are not affected. Ectopic expression of rough in the R7 precursor, through the use of the sevenless promoter, causes this cell to develop into an R1-6 photoreceptor subtype; however, this cell still requires sevenless function for its neural differentiation. Taken together with previous analyses of the rough mutant phenotype, these results suggest that the normal role of rough is to establish the unique cell identity of photoreceptors R2 and R5.

The Notch locus and the genetic circuitry involved in early Drosophila neurogenesis

The genetic and molecular analysis of the Notch locus, which codes for a transmembrane protein sharing homology with the mammalian epidermal growth factor, suggests that the Notch protein is involved in a cell interaction mechanism essential for the differentiation of the embryonic nervous system of Drosophila. Taking advantage of the negative complementation between two Notch mutations that affect the extracellular domain of the protein, we have tried to dissect the genetic circuitry in which Notch is integrated by searching for genes whose products may interact with the Notch protein. This genetic screen has led to the identification of a surprisingly restricted set of interacting loci, including Delta and mastermind. Like Notch, both of these genes belong to a group of loci, the neurogenic loci, which have been previously identified by virtue of their similar mutant phenotype affecting early neurogenesis. We extend these studies by systematically exploring interactions between specific mutations in the Notch molecule and the other neurogenic genes. Furthermore, we show that the molecular lesions of two Notch alleles (nd and nd2), which interact dramatically with mastermind mutations, as well as with a mutation affecting the transducin homologous product of the neurogenic locus Enhancer of split, involve changes in the intracellular domain of the protein.