Combgap relays wingless signal reception to the determination of cortical cell fate in the Drosophila visual system.

The dorsoventral axis of the Drosophila visual cortex is patterned by nonautonomous signals expressed at its dorsal and ventral margins. wingless (wg) expression at the margins induces decapentaplegic (dpp), optomotor blind (omb), and aristaless in adjacent domains. We show that Combgap, a zinc finger protein, represses Wg target gene expression in the visual cortex. Wg signal reception downregulates combgap expression and derepresses target gene transcription. Combgap participates in a Hedgehog-controlled circuit in the developing wing and leg by regulating the expression of Cubitus interruptus. Combgap is thus a tissue-specific relay between Wingless and its target genes for the determination of cell fate in the visual cortex.

Axonal signals in the assembly of neural circuitry.

Recent work in Drosophila and rodents has revealed that proteins transported along axons and delivered to pathway and target cell populations play important roles in the construction of neural circuitry. Interestingly, the parallels between these systems may extend to the identities of some of the molecules involved.

A retinal axon fascicle uses spitz, an EGF receptor ligand, to construct a synaptic cartridge in the brain of Drosophila.

Photoreceptor axons arriving in the Drosophila brain organize their postsynaptic target field into a precise array of five neuron "cartridge" ensembles. Here we show that Hedgehog, an initial inductive signal transported along retinal axons from the developing eye, induces postsynaptic precursor cells to express the Drosophila homolog of the epidermal growth factor receptor (EGFR). The EGFR ligand Spitz, a signal for ommatidial assembly in the compound eye, is transported to retinal axon termini in the brain where it acts as a local cue for the recruitment of five cells into a cartridge ensemble. Hedgehog and Spitz thus bring about the concerted assembly of ommatidial and synaptic cartridge units, imposing the "neurocrystalline" order of the compound eye on the postsynaptic target field.

Signals transmitted along retinal axons in Drosophila: Hedgehog signal reception and the cell circuitry of lamina cartridge assembly.

The arrival of retinal axons in the brain of Drosophila triggers the assembly of glial and neuronal precursors into a 'neurocrystalline' array of lamina synaptic 'cartridges'. Hedgehog, a secreted protein, is an inductive signal delivered by retinal axons for the initial steps of lamina differentiation. In the development of many tissues, Hedgehog acts in a signal relay cascade via the induction of secondary secreted factors. Here we show that lamina neuronal precursors respond directly to Hedgehog signal reception by entering S-phase, a step that is controlled by the Hedgehog-dependent transcriptional regulator Cubitus interruptus. The terminal differentiation of neuronal precursors and the migration and differentiation of glia appear to be controlled by other retinal axon-mediated signals. Thus retinal axons impose a program of developmental events on their postsynaptic field utilizing distinct signals for different precursor populations.

Hedgehog, transmitted along retinal axons, triggers neurogenesis in the developing visual centers of the Drosophila brain.

The development of the visual centers of the Drosophila brain is tightly regulated by the ingrowth of retinal axons from the developing eye. In the first optic ganglion, the lamina, arriving retinal axons trigger the precursors of their synaptic partners to complete a final cell division and commence neural differentiation. The secreted product of the hedgehog gene regulates the temporal assembly of photoreceptor precursor cells into ommatidial clusters in the compound eye. Here, we show that Hedgehog is transmitted along the retinal axons to serve as the inductive signal in the brain. Hedgehog acts in the first of two retinal axon-mediated steps in the assembly of lamina synaptic cartridges. These observations provide a novel insight into the molecular interactions that orchestrate the assembly of neural precursor cells into precise synaptic circuits.

Pattern formation in the visual centers of the Drosophila brain: wingless acts via decapentaplegic to specify the dorsoventral axis.

A stepwise morphogenetic program of cell division and cell fate determination generates the precise neuronal architecture of the visual centers of the Drosophila brain. Here, we show that the assembly of the target structure for ingrowing retinal axons involves cell-cell interactions mediated by the secreted product of the wingless (wg) gene. wg, expressed in two symmetrical domains of the developing brain, is required to induce and maintain the expression of the secreted decapentaplegic (dpp) gene product in adjacent domains. wg and dpp function are required for target field neurons to adopt their proper fates and to send axons into the developing target structure. These observations implicate a cascade of diffusible signaling molecules in patterning the visual centers of the Drosophila brain.

Topography in the Drosophila visual system.

The Drosophila visual system offers an excellent opportunity for studying the development of proper retinotopic connections at the level of individual identifiable cell types. Recent work suggests that, despite obvious anatomical and developmental differences, at least some of the general developmental strategies operating in the Drosophila visual system parallel observations made previously for vertebrates. The extensive repertoire of powerful genetic and molecular techniques available in Drosophila can now be directed towards determining whether these parallels also reflect similarities in the underlying molecular mechanisms.

Independent guidance of retinal axons in the developing visual system of Drosophila.

The development of the adult visual system of Drosophila requires the establishment of precise retinotopic connections between retinal photoreceptor cell axons and their synaptic partners in the optic lobe of the brain. To assess the role of axon-axon interactions in retinal axon guidance, we used genetic methods to disrupt the normal spatiotemporal order of retinal axon ingrowth. We examined retinal axon projections to the developing first optic ganglion, the lamina, in two mutants in which reduced numbers of ommatidia develop in the eye imaginal disk. We find that in the developing lamina of these mutants, sine oculis and Ellipse, retinal axons project to proper dorsoventral positions despite the absence of the usual array of neighboring retinal axons. In a second approach, we examined animals that were somatic mosaics for the mutation, glass. In glass- animals, retinal axons project aberrantly and the larval optic nerve is absent. We find that in the developing lamina of glass mosaic animals, wild-type retinal axons project to proper dorsoventral positions despite the misrouted projections of neighboring glass- retinal axons. In addition, wild-type retinal axons project normally in the absence of the larval optic nerve, indicating that the latter is not an essential pioneer for retinal axon navigation. Our observations support the proposal that axon fascicles can make at least some pathfinding decisions independently of other retinal axon fascicles. We suggest that positional guidance cues that might label axon pathways and target destinations contribute to retinotopic pattern formation in the Drosophila visual system.

Ablation of Drosophila photoreceptor cells by conditional expression of a toxin gene.

We have used toxin-mediated ablation to study some aspects of visual system development in Drosophila melanogaster. To devise a method that permits the conditional expression of a cellular toxin, we introduced an amber mutation into the diphtheria toxin-A-chain gene. In transgenic animals, this toxin gene can be activated by providing the gene for an amber suppressor tRNA. By coupling this toxin gene to the photoreceptor cell-specific promoter of the chaoptic gene, photoreceptor cells could be specifically ablated during development. Photoreceptor cell-specific markers normally activated during pupal development failed to appear after midpupation. Photoreceptor cells were absent from the retinas of adult flies at eclosion. We have assessed the consequences of photoreceptor cell ablation for eye and optic lobe development. We suggest that the larval photoreceptor nerve is not essential, in the late larval stages, for retinula photoreceptor cell axons to achieve their proper projection pattern in the brain. Moreover, while retinula photoreceptor innervation is initially required for the development of normal optic ganglia, the ablation of these cells in midpupation has no discernible effect. This approach to cell-specific ablation should be generally applicable to the study of cellular functions in development and behavior.

Synapsis-mediated fusion of free DNA ends forms inverted dimer plasmids in yeast.

When yeast (Saccharomyces cerevisiae) is transformed with linearized plasmid DNA and the ends of the plasmid do not share homology with the yeast genome, circular inverted (head-to-head) dimer plasmids are the principal product of repair. By measurements of the DNA concentration dependence of transformation with a linearized plasmid, and by transformation with mixtures of genetically marked plasmids, we show that two plasmid molecules are required to form an inverted dimer plasmid. Several observations suggest that homologous pairing accounts for the head-to-head joining of the two plasmid molecules. First, an enhanced frequency of homologous recombination is detected when genetically marked plasmids undergo end-to-end fusion. Second, when a plasmid is linearized within an inverted repeat, such that its ends could undergo head-to-tail homologous pairing, it is repaired by intramolecular head-to-tail joining. Last, in the joining of homologous linearized plasmids of different length, a shorter molecule can acquire a longer plasmid end by homologous recombination. The formation of inverted dimer plasmids may be related to some forms of chromosomal rearrangement. These might include the fusion of broken sister chromatids in the bridge-breakage-fusion cycle and the head-to-head duplication of genomic DNA at the sites of gene amplifications.

Fine structure recombinational analysis of cloned genes using yeast transformation.

We describe a general method for analyzing the genetic fine structure of plasmid-borne genes in yeast. Previously we had reported that a linearized plasmid is efficiently rescued by recombination with a homologous restriction fragment when these are co-introduced by DNA-mediated transformation of yeast. Here, we show that a mutation can be localized to a small DNA interval when members of a deletion series of wild-type restriction fragments are used in the rescue of a linearized mutant plasmid. The resolution of this method is to at least 30 base pairs and is limited by the loss of a wild-type marker with proximity to a free DNA end. As a means for establishing the nonidentity of two mutations, we determined the resolution of two-point crosses with a mutant linearized plasmid and a mutant homologous restriction fragment. Recombination between mutations separated by as little as 100 base pairs was detected. Moreover, the results indicate that exchange within a marked interval results primarily from one of two single crossovers that repair the linearized plasmid. These approaches to mapping the genetic fine structure of plasmids should join existing methods in a robust approach to the mutational analysis of gene structure in yeast.

Plasmid construction by homologous recombination in yeast.

We describe a convenient method for constructing new plasmids that relies on interchanging parts of plasmids by homologous recombination in Saccharomyces cerevisiae. A circular recombinant plasmid of a desired structure is regenerated after transformation of yeast with a linearized plasmid and a DNA restriction fragment containing appropriate homology to serve as a substrate for recombinational repair. The free ends of the input DNA molecules need not be homologous in order for efficient recombination between internal homologous regions to occur. The method is particularly useful for incorporating into or removing from plasmids selectable markers, centromere or replication elements, or particular alleles of a gene of interest. Plasmids constructed in yeast can subsequently be recovered in an Escherichia coli host. Using this method, we have constructed an extended series of new yeast centromere, episomal and replicating (YCp, YEp, and YRp) plasmids containing, in various combinations, the selectable yeast markers LEU2, HIS3, LYS2, URA3 and TRP1.

Transformation of yeast with linearized plasmid DNA. Formation of inverted dimers and recombinant plasmid products.

The molecular products of DNA double strand break repair were investigated after transformation of yeast (Saccharomyces cerevisiae) with linearized plasmid DNA. DNA of an autonomous yeast plasmid cleaved to generate free ends lacking homology with the yeast genome, when used in transformation along with sonicated non-homologous carrier DNA, gave rise to transformants with high frequency. Most of these transformants were found to harbor a head-to-head (inverted) dimer of the linearized plasmid. This outcome of transformation contrasts with that observed when the carrier DNA is not present. Transformants occur at a much reduced frequency and harbor either the parent plasmid or a plasmid with deletion at the site of the cleavage. When the linearized plasmid is introduced along with sonicated carrier DNA and a homologous DNA restriction fragment that spans the site of plasmid cleavage, homologous recombination restores the plasmid to its original circular form. Inverted dimer plasmids are not detected. This relationship between homologous recombination and a novel DNA transaction that yields rearrangement could be important to the cell, as the latter could lead to a loss of gene function and lethality.

Formation of inverted dimer plasmids after transformation of yeast with linearized plasmid DNA.

The formation of an inverted dimer plasmid on transformation with linear molecules is formally analogous to the fusion of the daughters of a broken chromosome at their broken ends. In the latter case, this leads to the formation of a dicentric chromosome, which could break at anaphase. Hence the process is cyclic. Similarly, when our linear molecules are modified by the addition of a cloned yeast centromere, dicentric inverted dimers are not obtained. Instead, we obtain monocentric plasmids with partial duplication and deletion that apparently derive from a process of fusion, bridge-breakage, and fusion. This is not surprising, since it is known that dicentric plasmids undergo breakage in yeast (Mann and Davis 1983). However, any apparent similarity of this process to that which occurs with a broken chromosome in maize must be tempered by the special nature of the transformation process. Most significantly, inverted dimers are rare when sonicated carrier DNA is not present during the transformation. This requirement is not understood, but it is a condition that may not be met in a yeast cell harboring a broken chromosome. It is possible that carrier DNA induces a repair process that results in fusion. On the other hand, a property of the transformation process that results in an inhibition of fusion may be overcome by the presence of carrier DNA. Most inverted dimers are apparently formed from an interaction between two input linear molecules. We cannot rule out the possibility that a minor fraction derive from a single molecule. Thus, the fusion of two input molecules is a much more efficient process than a replicative process that could occur with single linear molecule. For a similar fusion process to occur with a broken yeast chromosome, replication would be required. We do not know if a broken yeast chromosome can replicate. Evidence consistent with the presence of a breakage-fusion-bridge process in yeast has been obtained through the formation of dicentric chromosomes via meiotic recombination (Haber et al. 1984). Spores from these meioses sometimes give rise to a clone that is mixed for markers of the chromosome that could have been dicentric. A process of fusion-bridge-breakage could account for the formation of some of these mixed clones. However, the dicentric chromosomes apparently often survive meiotic disjunction and break in the spore's first mitotic anaphase or possibly in a later generation. Thus, the interpretation of the origin of these mixed clones is uncertain. Some aspects of the fusion process are especially intriguing.(ABSTRACT TRUNCATED AT 400 WORDS)

Structure of chi hotspots of generalized recombination.

Chi recombinational hotspots are sites around which the rate of Rec-promoted recombination in bacteriophage lambda is elevated. Examination of a derivative of lambda into which the plasmid pBR322 was inserted reveals that pBR322 lacks Chi sites. Using this lambda-pBR322 hybrid, we obtained mutations creating Chi sites at three widely separated loci within pBR322. Nucleotide sequence analysis reveals that the mutations are single base-pair changes creating the octamer 5' GCTGGTGG 3'. This sequence is present at three previously analyzed Chi sites in lambda, and all analyzed mutations creating or inactivating these Chi sites occur within this octamer. We conclude that Chi is 5' GCTGGTGG 3', or its complement, or both.