A multifaceted approach to neural development

Positional information and opsin identity in retinal cones

To test the hypothesis that local environmental cues regulate the expression of middle wavelength-sensitive (MWS) and short wavelength-sensitive (SWS) opsins in cone photoreceptors, we examined the development of the neonatal mouse retina in an organotypic culture system. The segregation of MWS and SWS cones into dorsal and ventral fields in the mouse retina offers an opportunity to isolate a phenotypically homogeneous population of immature cones prior to opsin expression. Retinae were harvested from mice ranging in age from birth (P0) to P18 and maintained in vitro for up to 4 weeks. Cones from newborn mice were first immunoreactive to SWS opsin-specific antibodies (OS-2 and JH455) after 5 days in vitro, which corresponds to a time course similar to that in vivo. The topographic separation of SWS cones into distinct dorsal and ventral fields was also obvious in retinal explants from newborn mice. However, the MWS opsin, identified by polyclonal antibody JH492, was expressed only in vitro when dorsal explants were harvested from P3 or older pups. Despite the absence of MWS opsin expression in newborn retinal cultures, there was no evidence of an increase in the numbers of SWS cones. To test if local diffusable cues could induce immature cones to express an aberrant opsin, dorsal and ventral retinal explants at different stages of maturation were cocultured during the incubation period. Neither the emergence of the cone fields nor the difference in the regional and temporal development of the MWS and SWS opsins was affected in these experiments. These results suggest that positional information in the retina and the opsin identity of cones is determined prior to birth and argue against the hypothesis that postnatal cones can be induced to express an aberrant opsin.

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.

Mad acts downstream of Dpp receptors, revealing a differential requirement for dpp signaling in initiation and propagation of morphogenesis in the Drosophila eye

Decapentaplegic (Dpp), a member of the TGF-betta family of cytokines, has been implicated in many patterning processes in Drosophila, including the initial steps of pattern formation in the developing eye. We show that the Mothers against dpp (Mad) gene is required for dpp signaling during eye development. Clonal analysis demonstrates a cell-autonomous function for Mad and genetic interactions indicate that Mad is an essential component of the signal transduction pathway downstream of the Dpp receptors in responding cells. Mad-mediated dpp signaling is absolutely required for the initiation of the morphogenetic furrow in the eye, but has only a minor role in its subsequent propagation across the eye

Genetic interactions of pokkuri with seven in absentia, tramtrack and downstream components of the sevenless pathway in R7 photoreceptor induction in Drosophila melanogaster

The sevenless (sev) cascade plays an inductive role in formation of the R7 photoreceptor, whilst the pokkuri (pok) and tramtrack (ttk) gene products are known to repress R7 induction in developing ommatidia of Drosophila melanogaster. To elucidate how these positive and negative signalling mechanisms co-operate in the normal fate determination of R7, genetic interactions of mutations in the pok locus with ttk and downstream elements of sev including Gap1, raf1, rolled (r1) and seven in absentia (sina) were examined. The eye phenotype of a weak hypomorph, pok ¹⁵, was enhanced dominantly by Gap1-^mip, a recessive mutation in a gene encoding a down-regulator of Ras1, producing multiple R7 in ommatidia. Ras1 has been reported to activate r1-encoded mitrogen-activated protein (MAP) kinase via Raf1 that is associated physically with Rasl. Ommatidia of raf1 ^c110 and rl ²/rl^EMS64 typically lacked R7 and a few outer photoreceptors. The pok ¹ mutation suppressed dominantly the rafl ^c110 rl²/rl^EMS64 eye phenotypes, allowing single R7 cells to develop in ommatidia. The rafl ^c110 mutation improved adult viability of pok ¹ homozygotes. An in vitro experiment demonstrated that MAP kinase phosphorylates Pok protein. Ttk is a transcriptional repressor which binds to the regulatory sequence upstream of the fushi-tarazu (ftz), even skipped (eve) and engrailed (en) coding region. A reduced activity in ttk resulted in enhancement of the pok phenotype. ttk mutations produced extra R7 cells even in sina homozygotes whilst the pok mutation did not. This result indicates that Ttk represses R7 induction downstream of the sites where Pok and Sina function.

Green fluorescent protein and its derivatives as versatile markers for gene expression in living Drosophila melanogaster, plant and mammalian cells

We have investigated the utility of the green fluorescent protein (GFP) as a marker for gene expression in living adult Drosophila melanogaster (Dm) and cultured plant and mammalian cells. Using Dm, we generated transgenic flies bearing a glass-responsive gfp fusion gene to test the utility of GFP as a spatial reporter. In the adult living fly, GFP is clearly visible in the ocelli and the eye. We have optimized the use of filters for distinguishing the GFP signal from abundant autofluorescence in living Dm. In addition, we have used GFP to identify photoreceptor cells in pupal eye cultures that have been fixed and stained according to standard histological procedures. GFP was also detected in individual living plant cells following transient transfection of soybean suspension cultures, demonstrating that GFP is an effective transformation marker in plant cells. Similarly, transient transfection of mammalian cells with a modified form of GFP, S65T, allowed detection of single living cells expressing the reporter. This modified form of GFP gave a robust signal that was resistant to photobleaching. We then used a CellScan system exhaustive photon reassignment (EPR) deconvolution algorithm to generate high-resolution three-dimensional images of GFP fluorescence in the living cell.

Establishment of Drosophila imaginal precursor cells is controlled by the Arrowhead gene

Metamorphosis in Drosophila melanogaster requires synchronization of numerous developmental events that occur in isolated imaginal precursor tissues. The imaginal primordia are established during embryonic stages and are quiescent for much of larval life. The Arrowhead gene is necessary for establishment of proper numbers of cells within a subset of imaginal precursor tissues. Loss-of-function mutations in Arrowhead reduce the number of abdominal histoblasts and salivary gland imaginal ring cells before the proliferative stages of their development. The number of abdominal histoblasts in mutant animals is approximately half that of wild-type, as might result from failure of a single early division of these cells. A neomorphic Arrowhead allele results in the specific loss of the retinal precursors by the early third instar, before they have begun to differentiate. Since Arrowhead mutations affect only subsets of imaginal tissue, there must be distinctions in the developmental regulation of different imaginal precursors. Arrowhead may be part of a regulatory pathway responsible for establishing the proper number of abdominal histoblasts and salivary gland imaginal ring cells. The neomorphic Arrowhead allele, which may cause misexpression of the Arrowhead gene in the eye-antenna imaginal disc, interferes with the establishment or proliferation of retinal precursor cells.

Circadian rhythms in Drosophila can be driven by period expression in a restricted group of central brain cells

Neural tissues controlling circadian rhythmicity have been identified in a variety of organisms and are often closely associated with the visual system. In Drosophila, the clock gene period (per), which is required for circadian rhythms, is expressed in many neurons and glia throughout the eye and brain. We asked whether biological rhythms could be generated if per expression were restricted to a subset of these cells that is involved in photoreception. Here we demonstrate that expression of per under the control of the glass promoter confers both behavioral and molecular rhythmicity. glass is required for development of Drosophila photoreceptors, and this promoter is active in eyes, ocelli, and certain cells of the central brain. When we genetically removed all external photoreceptor cells, rhythms persisted in these transgenic animals. This suggests that a few central brain cells producing glass and per are capable of generating biological rhythms.

Role of the proneural gene, atonal, in formation of Drosophila chordotonal organs and photoreceptors

The Drosophila gene atonal encodes a basic helix-loop-helix protein similar to those encoded by the proneural genes of the achaete-scute complex (AS-C). The AS-C are required in the Drosophila PNS for the selection of neural precursors of external sense organs. We have isolated mutants of atonal, which reveal that this gene encodes the proneural gene for chordotonal organs and photoreceptors. In atonal mutants, all observable adult chordotonal organs, and almost all embryonic chordotonal organs fail to form; all adult photoreceptors are missing. For both types of sense organ, this defect is already apparent at the level of precursor formation. Therefore it is a failure in the epidermal-neural decision process i.e. a proneural defect. The failure to form photoreceptors results in atrophy of the atonal mutant imaginal disc, due to apoptosis and lack of stimulation of division. Lack of photoreceptors should also eliminate signalling that arises from differentiating photoreceptors and is required for morphogenetic furrow movement in the wild-type eye disc. Nevertheless, a remnant morphogenetic furrow is still observed in the atonal mutant disc. This presumably reflects the process of furrow initiation, which would not depend on signals from developing photoreceptors.

Model of forebrain regionalization based on spatiotemporal patterns of POU-III homeobox gene expression, birthdates, and morphological features

In situ hybridization was used to map spatiotemporal expression patterns of the four known intronless POU-III transcription factor genes Brn-1, Brn-2, Brn-4, and Tst-1 in the developing rat forebrain vesicle, beginning on embryonic day 10. The results indicate that the proliferation layers (ventricular and subventricular) and mantle layer of the forebrain neural tube each display a strikingly unique pattern of regionalized POU-III expression. Within a particular region, or layer within a region, none to all four of the mRNAs may be detected, and during development a particular mRNA in a particular region displays one of five expression patterns, or a combination of these patterns, which may be described as conserved, lost, transient, acquired, or redeployed expression. In the developing brain as a whole, Brn-1 and Brn-2 early on display somewhat different spatial expression patterns that converge to essential identity in the adult, whereas Brn-4 expression is initially broad and becomes much more restricted in the adult, and Tst-1 expression expands greatly through development. Usually, though not always, expression patterns tend to correlate with major histological features in the forebrain (often internal or external sulci associated with proliferation zones), and little evidence for waves of expression moving through the whole forebrain over time was obtained. Thus, clear differences in hybridization intensity often are observed between the cerebral cortex, basal telencephalic nuclei, hypothalamus, ventral thalamus, dorsal thalamus, and pretectal region. In contrast, transverse bands of hybridization extending from the roof to the floor of the forebrain, corresponding to proposed neuromeres, were not observed with these probes. The results suggest that POU-III transcription factors help define specific regions in the early neuroepithelium as well as different cellular phenotypes in the ventricular, subventricular, and mantle layers of specific regions later in development. Thus, the functions of these regulatory proteins may be different in proliferating neuroepithelial cells, young neurons, and mature neurons and appear to be region-specific.

Relationships between complex Delta expression and the specification of retinal cell fates during Drosophila eye development

Analysis of retinal development in Delta (Dl) temperature-sensitive mutants reveals requirements for Delta function in the specification of all retinal cells, including photoreceptors, cone cells, pigment cells and cells that make up interommatidial bristles. In situ hybridization and immunohistochemistry indicate that Delta is expressed dynamically during the specification of different cell types. Comparisons of Delta expression patterns with developmental defects in Dl mutants implies that Delta functions in a cell-nonautonomous manner in the specification of photoreceptors. Delta protein resides predominantly in subcellular vesicles located primarily at the apical ends of developing retinal cells. Localization of Delta protein in Dl and shibire tsl mutants implies that Delta is targeted to the cell surface, but is efficiently removed via endocytosis, resulting in vesicular accumulation.

canoe encodes a novel protein containing a GLGF/DHR motif and functions with Notch and scabrous in common developmental pathways in Drosophila

The canoemisty1 (cnomis1) mutation was isolated by virtue of its severe rough eye phenotype from approximately 500 fly lines, each harboring a single autosomal insertion of a P element (Bm delta w). Excision of the P element generated a lethal, null allele, cnomis10, together with many revertants with normal eye morphology. Ommatidia homozygous for cnomis10, produced in an otherwise wild-type eye by somatic recombination, typically contain a reduced number of outer photoreceptors. Some cnomis1 homozygous adults bear extra macrochaetes on the head, notum, humerus and/or scutellum. cnomis1 hemizygotes often show conspicuous wing phenotypes such as a notched blade and the loss of a cross vein. The sequence of cno cDNA clones isolated from an embryonic cDNA library revealed a long open reading frame that potentially encodes a 1893-amino-acid protein with the GLGF/DHR motif, a conserved sequence in Discs large, Dishevelled, and some other proteins associated with cellular junctions. Flies doubly mutant for cnomis1 and scabrous1 (sca1) and those for cnomis1 and the split (spl) allele of Notch (N) always have rumpled wings curved downward. The spl; cnomis1 double mutant flies also exhibit a "giant socket" phenotype. These phenotypes are rarely observed flies singly mutant for either cnomis1, sca1 or spl. The wing vein gaps caused by Abruptex1, a N allele producing an activated form of N protein, are dominantly suppressed by cnomis1. Heterozygosity for shaggy and myospheroid promotes formation of extra wing veins in cnomis1 homozygotes. The genetic interactions suggest that cno participates with members of the N pathway in regulating adhesive cell-cell interactions for the determination of cell fate.

Optomotor-blind of Drosophila melanogaster: a neurogenetic approach to optic lobe development and optomotor behaviour

The gene optomotor-blind (omb) plays a crucial role in Drosophila optic lobe development. Various mutations in omb lead to different structural defects in the adult optic lobes with correlated behavioural phenotypes. Molecular analysis of omb allows one to trace back behavioural defects to the spatio-temporal misexpression of the gene in mutant development.

Organ and cell allometry in Hawaiian Drosophila: how to make a big fly

The importance of body size in predicting many aspects of an animal's biology has become well established in recent years. However, little is known about how body size evolves at the cellular level. Some published data suggest that it is cell number and not cell size that accompanies changes in organ and body size across taxa. We examined organ and cell allometry in the wing, eye and basitarsus of adult Hawaiian Drosophila, ranging in body length from 0.2 mm to 0.8 mm. Linear measurements of all three structures exhibit a positive allometry with body length. Exponents of the allometric equation were 0.96, 0.55 and 1.50 for wing, eye and basitarsus, respectively. Surface markers were used to quantify cell size of each organ. The allometric exponents for cell size as a function of organ size were 0.53, 0.68 and 0.33 for wing, eye and basitarsus, respectively. In contrast to reports in the literature on other systems, our results for Hawaiian Drosophila indicate that cell size may contribute between one third and two thirds to evolutionary changes in organ and body size.

Genetic mosaic analysis of the equatorial-less mutation in Drosophila melanogaster

eql (equatorial-less) is a recessive lethal mutation on the second chromosome of Drosophila melanogaster. J. Campos-Ortega found that eql clones in somatic mosaic flies have reduced numbers of photoreceptor cells, and he suggested that only the R1, R6, and R7 photoreceptor cells were missing in this mutant. These photoreceptor cells help to define the inverted orientation of ommatidial facets along the equatorial midline of the fly eye, hence the mutation was named "equatorial-less." We have conducted a detailed analysis of the eql mutation, by serial section reconstruction of eql clones marked with bw- or w- in somatic mosaic flies. We found that all photoreceptor cell types (R1-R8) could be deleted by the eql mutation, and in rare cases the number of photoreceptor cells was increased. The apparent lack of photoreceptor cell type specificity was confirmed by our analysis of genetically mosaic facets, which indicated that no single photoreceptor cell, or subset of photoreceptor cells, was uniquely required to express eql+. Rather, eql appears to function in all photoreceptor cells, and possibly in all eye precursor cells. The distribution of photoreceptor cell numbers in w eql facets was consistent with the hypothesis that each photoreceptor cell was deleted independently of the others. The eql gene is located on the right arm of chromosome 2 at map location 2-104.5 +/- 0.7 and lies between the polytene chromosome bands 59D8 and 60A7.

The Drosophila giant lens gene plays a dual role in eye and optic lobe development: inhibition of differentiation of ommatidial cells and interference in photoreceptor axon guidance

The Drosophila giant lens (gil) gene encodes a secreted molecule which if absent leads to the recruitment of additional ommatidial cells normally eliminated by apoptosis. Heat induced ectopic gil expression leads to a reduction of ommatidial cells suggesting that gil is secreted by differentiating cells to prevent the development of an excess of cells of a given ommatidial cell type. A second important defect is the misrouting of photoreceptor axons in gil mutants. However, gil function is not required in photoreceptor axons for the establishment of proper connections. We propose that gil acts on the development of lamina cells preventing the correct differentiation of the target region of photoreceptor axons and therefore leading to an axon guidance phenotype.

The spitz gene is required for photoreceptor determination in the Drosophila eye where it interacts with the EGF receptor

Little is known about the mechanisms by which photoreceptors other than R7 are determined during Drosophila eye development. By looking for mutations that modify the phenotype caused by ectopic expression of the rhomboid gene in the eye, I have discovered that the spitz gene is required for photoreceptor determination. Mosaic analysis suggests that spitz, which encodes a TGF alpha homologue, produces a diffusible signal during ommatidial development. Other members of the spitz group and the EGF receptor also interact with sev-rho, in a pattern that suggests a model in which rhomboid can act as a mediator of a ligand-receptor interaction between spitz and Egfr in the developing eye. These data suggest that photoreceptors other than R7 use a Ras1 signalling pathway activated by the spitz/Egfr interaction, in a manner analogous to the Ras1 pathway activated by boss/sevenless in photoreceptor R7.

Drosophila Jun mediates Ras-dependent photoreceptor determination

The expression of the D. melanogaster transcription factor Jun in the eye imaginal disc correlates temporally and spatially with the determination of neuronal photoreceptor fate. Expression of dominant negative forms of Jun in photoreceptor precursor cells results in dose-dependent loss of photoreceptors in the adult fly. Conversely, localized overexpression of Jun in the eye imaginal disc can induce the differentiation of additional photoreceptor cells. Furthermore, the transformation of nonneuronal cone cells into R7 neurons elicited by constitutively active forms of sevenless, Ras1, Raf, and MAP kinase is relieved in the presence of Jun mutants. These results demonstrate a requirement of Jun downstream of the sevenless/ras signaling pathway for neuronal development in the Drosophila eye.

Marbles mutants: uncoupling cell determination and nuclear migration in the developing Drosophila eye

Morphogenesis of a multicellular structure requires not only that cells are specified to express particular gene products, but also that cells move to adopt characteristic shapes and positions. Little is known about how these two aspects of morphogenesis are coordinated. The developing Drosophila compound eye is a monolayer, in which cells are suspended between apical and basal membranes and assemble sequentially into hundreds of unit eyes, or facets, guided by a series of cell interactions. As cells are determined to join the facet, their nuclei and cell bodies rise apically and then settle into position in the cell group. The final nuclear positions determine the shape of the individual cells. We have identified a Drosophila gene called marbles which is required for the apical nuclear migrations that accompany cell determination during eye development. In marbles mutant eyes, the sequence of cell specification that leads to the formation of facets occurs almost normally despite the failure of nuclear migration in many cells. The marbles mutant phenotype reveals that during Drosophila eye development cell determination does not require nuclear migration.

Misexpression of the Drosophila argos gene, a secreted regulator of cell determination

I have examined the effects on cells in the developing eye of over-expressing the argos gene. Transgenic flies carrying argos expressed under hsp70 and sevenless control sequences were analysed. All cell types in the developing eye (except bristles) are sensitive to argos concentration: over-expression leads to too few cells forming, the opposite phenotype to that seen in argos loss-of-function mutants. This effect was only seen with HS-argos flies: sev-argos flies, which over-express the protein at a lower level are not affected, suggesting that a considerable over-expression is required to disrupt cell fate. However, sev-argos is able to rescue argos eye mutations completely, indicating that the normal expression pattern is not critical for wild-type eye development. By transfecting argos into tissue culture cells, I show that the protein is secreted in a soluble form.

Rotation of photoreceptor clusters in the developing Drosophila eye requires the nemo gene

The Drosophila eye consists of a reiterative hexagonal array of photoreceptor cell clusters, the ommatidia. During normal morphogenesis, the clusters in the dorsal or ventral halves of the disc rotate 90 degrees in opposite directions, forming mirror images across a dorsoventral equator. In the mutant nemo (nmo), there is an initial turning of approximately 45 degrees, but further rotation is blocked. Genetic mosaic analysis indicates that the nmo gene acts upon each cluster as a whole; normal nmo function in one or more photoreceptor cells appears to be sufficient to induce full rotation. The nmo gene sequence encodes a serine/threonine protein kinase homolog, suggesting that the kinase is required to initiate the second step of rotation. In another mutant, roulette, excessive rotation through varying angles occurs in many ommatidia. This defect is suppressed by nmo, indicating that nmo acts upstream in a rotation-regulating pathway.

An array of early differentiating cones precedes the emergence of the photoreceptor mosaic in the fetal monkey retina

We previously have demonstrated that approximately 10% of cones in the fetal monkey retina precociously express the red/green opsin. These data suggested the possibility that a subset of cones differentiates prior to their nascent cone neighbors. To further assess this early cone differentiation in the fetal monkey retina, we used monoclonal antibodies proven to be important developmental markers of photoreceptor phenotypes and synaptogenesis (XAP-1, specific to photoreceptor membranes; SV2, specific to synaptic vesicle protein). Although these two antibodies recognize functionally distinct antigens, our analyses revealed that both identify a subset of precociously immunoreactive cones. Further, XAP-1- and SV2-positive cones are distributed in the same pattern as precocious red/green-sensitive cones in immature regions of the fetal monkey retina. These results support the hypothesis that the primate retina possesses a spatially organized protomap that may induce the emergence of the photoreceptor mosaic and trigger the formation of color-specific pathways that include horizontal, bipolar, and retinal ganglion cells.

Confrontation of scabrous expressing and non-expressing cells is essential for normal ommatidial spacing in the Drosophila eye

The establishment of neural precursor cells in Drosophila depends on cell-cell interactions and lateral inhibition. Scabrous (sca) is involved in this process by preventing an excess of cells from adopting a neural precursor fate. Specifically in eye development, Sca protein function has been implicated in the spacing pattern that is essential for the ordered appearance of the ommatidial array. During this process sca expression is restricted to neurogenic groups of cells and later to the neural precursors. We report that ectopic sca expression in the morphogenetic furrow results in a rough eye phenotype with oversized and fused ommatidia. These defects in adult eyes are due to the generation of too many ommatidial preclusters in the morphogenetic furrow. Strikingly, sca loss-of-function mutants have an almost identical phenotype. Our results suggest that Sca plays a positive role in establishing the spacing pattern within the furrow and that the quantitative difference in sca expression between neighboring groups of cells is a determining factor in this process. Ectopic expression of Sca also represses endogenous sca expression in the furrow, suggesting that Sca is involved in a feedback loop affecting its own transcription. Interestingly, sca shares homology to a group of extracellular matrix proteins that have been implicated in neuronal differentiation. We present a model for sca function based on its phenotypic and molecular features.

Atonal is the proneural gene for Drosophila photoreceptors

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.

Signaling mechanisms in induction of the R7 photoreceptor in the developing Drosophila retina

The Drosophila compound eye is an excellent experimental system for analysing fate induction of identifiable single cells. Each ommatidium, a unit eye, contains eight photoreceptors (R1-R8), and the differentiation of these photoreceptors occurs in the larval eye imaginal disc in discrete steps: first R8 is determined, then R2/R5, R3/R4, R1/R6 and finally R7. Induction of R7, in particular, has been extensively studied at the molecular level. The R8 photoreceptor presents on its surface a ligand, Bride of Sevenless, that binds and activates Sevenless receptor tyrosine kinase in the R7 precursor. Autophosphorylated Sevenless initiates a Ras1-mediated cascade, which eventually activates transcription factors in the nucleus via Raf1 and MAP kinases, resulting in R7 development. However, recent studies indicate that Sevenless (Sev) functions just to neuralize the cell and has no role in R7 fate determination per se. It appears that the R7 fate may represent the lowest rung of a 'neuronal ground state', which is attained without any specific inductive cue. It is plausible that the R7 precursor is actively prevented from taking on the neuronal fate and this inhibition is removed by activation of Sev.

Migration of glia along photoreceptor axons in the developing Drosophila eye

We have identified a set of retinal basal glia, designated RBG cells, in the axon layer of the developing Drosophila eye disc. In vivo pulse labeling with bromodeoxyuridine shows that these cells originate in the optic stalk and migrate into the disc. In mutants lacking photoreceptor axons, RBG cells accumulate in the optic stalk, but do not invade the disc. The association of RBG cells with photoreceptor axons, their origin in the optic stalk, and their migration into the retina are in common with the behavior of astrocytes in the developing mammalian retina.

The TGF beta homolog dpp and the segment polarity gene hedgehog are required for propagation of a morphogenetic wave in the Drosophila retina

Development of the Drosophila retina occurs asynchronously; differentiation, its front marked by the morphogenetic furrow, progresses across the eye disc epithelium over a 2 day period. We have investigated the mechanism by which this front advances, and our results suggest that developing retinal cells drive the progression of morphogenesis utilizing the products of the hedgehog (hh) and decapentaplegic (dpp) genes. Analysis of hh and dpp genetic mosaics indicates that the products of these genes act as diffusible signals in this process. Expression of dpp in the morphogenetic furrow is closely correlated with the progression of the furrow under a variety of conditions. We show that hh, synthesized by differentiating cells, induces the expression of dpp, which appears to be a primary mediator of furrow movement.

Ectopic expression of seven-up causes cell fate changes during ommatidial assembly

During Drosophila ommatidial development, a single cell is selected within the ommatidial cluster to become the R7 photoreceptor neuron. The seven-up gene has been shown to play a role in this process by preventing four other photoreceptor precursors, R3/R4/R1/R6, from adopting the R7 cell fate. The seven-up gene encodes a steroid receptor-like molecule that is expressed only in those four cells that require seven-up function in the developing Drosophila ommatidium. We have examined the functional significance of the spatially restricted expression of seven-up by misexpressing seven-up isoforms. As expected from the function that seven-up performs in R3/R4/R1/R6, ubiquitous expression of seven-up causes transformation of the R7 cell to an R1-R6 cell fate. In addition, depending on the timing and spatial pattern of expression, various other phenotypes are produced including the loss of the R7 cell and the formation of extra R7 cells. Ubiquitous expression of seven-up close to the morphogenetic furrow interferes with R8 differentiation resulting in failure to express the boss protein, the ligand for the sevenless receptor tyrosine kinase, and the R7 cell is lost consequently. Extra R7 cells are formed by recruiting non-neuronal cone cells as photoreceptor neurons in a sevenless and bride of sevenless independent way. Thus, the spatiotemporal pattern of seven-up expression plays an essential role in controlling the number and cellular origin of the R7 neuron in the ommatidium. Our results also suggest that seven-up controls decisions not only between photoreceptor subtypes, but also between neuronal and non-neuronal fates.

Neuronal development in the Drosophila retina

Nervous systems of higher organisms are comprised of a variety of cell types which are interconnected in a precise manner. The molecular mechanisms that lead to the specification of neuronal cell types are not well understood. The compound eye of the fruit fly Drosophila is an attractive experimental system to understand these mechanisms. The compound eye is a reiterated neural pattern with several hundred unit structures and is amenable to both classical and molecular genetic methods. During the development of the compound eye cell-cell interactions and positional information play a critical role in the determination of cell fate. Recent genetic and molecular studies have provided important clues regarding the nature of the molecules involved in cellular signalling and neuronal differentiation.

Molecules and cognition: the latterday lessons of levels, language, and lac. Evolutionary overview of brain structure and function in some vertebrates and invertebrates

The characteristics of the nervous systems of a number of organisms in different phyla are examined at the recombinant DNA, protein, neuroanatomic, neurophysiological, and cognitive levels. Among the invertebrates, special attention is paid to the advantages as well as the shortcomings of the fly Drosophila melanogaster, the worm Caenorhabditis elegans, the honey bee Apis mellifera, the sea hare Aplysia californica, the octopus Octopus vulgaris, and the squid Loligo pealei. Among vertebrates, the focus is on Homo sapiens, the mouse Mus musculus, the rat Rattus norvegicus, the cat Felis catus, the macaque monkey Macaca fascicularis, the barn owl Tyto alba, and the zebrafish Brachydanio rerio. Vertebrate nervous systems have also been compared in fossil vs. extant organisms. I conclude that complex nervous systems arose in the Early Cambrian via a big bang that was underpinned by a modular method of construction involving massive pleiotropy of gene circuits. This rapidity of construction had enormous implications for the degrees of freedom that were subsequently available to evolving nervous systems. I also conclude that at the level of neuronal populations and interactions of neuropiles there is no model system between phyla except at the basic macromolecular level. Further, I argue that to achieve a significant understanding of the functions of extant nervous systems we need to concentrate on fewer organisms in greater depth and manipulate genomes via transgenic technologies to understand the behavioral outputs that are possible from an organism. Finally, I analyze the concepts of "perceptual categorization" and "information processing" and the difficulties involved in the extrapolation of computer analogies to sophisticated nervous systems.

Analysis of genetic mosaics in developing and adult Drosophila tissues

We have constructed a series of strains to facilitate the generation and analysis of clones of genetically distinct cells in developing and adult tissues of Drosophila. Each of these strains carries an FRT element, the target for the yeast FLP recombinase, near the base of a major chromosome arm, as well as a gratuitous cell-autonomous marker. Novel markers that carry epitope tags and that are localized to either the cell nucleus or cell membrane have been generated. As a demonstration of how these strains can be used to study a particular gene, we have analyzed the developmental role of the Drosophila EGF receptor homolog. Moreover, we have shown that these strains can be utilized to identify new mutations in mosaic animals in an efficient and unbiased way, thereby providing an unprecedented opportunity to perform systematic genetic screens for mutations affecting many biological processes.

The eyes absent gene: genetic control of cell survival and differentiation in the developing Drosophila eye

The eyes absent (eya) gene is required at an early stage in development of the D. melanogaster compound eye. In eya mutants, progenitor cells in the eye disc undergo programmed cell death anterior to the morphogenetic furrow, rather than proceeding into the pathway of retinal differentiation. A low level of cell death normally occurs at this stage, suggesting that eya activity influences the distribution of cells between differentiation and death. Molecular analysis identifies a nuclear protein expressed in progenitor cells prior to differentiation. Transformation with the cDNA prevents progenitor cell death and allows the events that generate the eye to proceed. eya activity is required for the survival of eye progenitor cells at a critical stage in morphogenesis.

Characterization of the radiation-sensitive stage in the development of the compound eye of Drosophila

The development of the compound eye of Drosophila is particularly sensitive to irradiation during the third larval instar. Moreover, the anterioposterior location of the eye-pattern defects produced by irradiation during the third instar is correlated with the age of the larvae at the time of irradiation, as first shown by H.J. Becker. The development of the fly eye proceeds from posterior to anterior, and so these results suggest that there may be a radiation-sensitive stage in the development of the precursor cells in the eye imaginal disc of Drosophila. We show here that irradiation of third-instar Drosophila larvae with 23-30 Gy of 60Co gamma-rays produces confluent pattern disruptions in a dorsoventral stripe of eye tissue with an average width of about 8 facets along the anterioposterior axis. By measuring the time interval from irradiation to pupariation in individual larvae, we were able to determine that the posterior boundary of the radiation-sensitive region is located 0-1 columns anterior to the morphogenetic furrow in the developing eye imaginal disc. Therefore the anterior boundary of the radiation-sensitive region lies about 8-9 columns anterior to the morphogenetic furrow. These boundaries demarcate the region of the eye imaginal disc within which a specific subset of precursor cells (those that will develop into the R1, R6 and R7 photoreceptor cells, as well as the pigment and cone cells) are preparing for their final round of mitosis. Irradiation of these precursor cells would cause the death or delayed mitosis of their daughter cells within the morphogenetic furrow, while they are initiating the cellular interactions that determine cell fate in the developing eye. Irradiation of more anterior cells (i.e., at earlier stages) results in few pattern defects, presumably because the resulting cell death and delayed mitosis can be completed before the morphogenetic furrow passes.

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.

Postembryonic patterns of expression of cut, a locus regulating sensory organ identity in Drosophila

The cut locus is both necessary and sufficient to specify the identity of a class of sensory organs in Drosophila embryos. It is also expressed in and required for the development of a number of other embryonic tissues, such as the central nervous system, the Malpighian tubules and the tracheal system. We here describe the expression of cut in the precursors of adult sensory organs. We also show that cut is expressed in cells of the prospective wing margin and correlate the wing margin phenotype caused by two cut mutations with altered cut expression patterns. Finally, we observe cut-expressing cells in other adult tissues, including Malpighian tubules, muscles, the central nervous system and ovarian follicle cells.

Identifying targets of the rough homeobox gene of Drosophila: evidence that rhomboid functions in eye development

In order to identify potential target genes of the rough homeodomain protein, which is known to specify some aspects of the R2/R5 photoreceptor subtype in the Drosophila eye, we have carried out a search for enhancer trap lines whose expression is rough-dependent. We crossed 101 enhancer traps that are expressed in the developing eye into a rough mutant background, and have identified seven lines that have altered expression patterns. One of these putative rough target genes is rhomboid, a gene known to be required for dorsoventral patterning and development of some of the nervous system in the embryo. We have examined the role of rhomboid in eye development and find that, while mutant clones have only a subtle phenotype, ectopic expression of the gene causes the non-neuronal mystery cells to be transformed into photoreceptors. We propose that rhomboid is a part of a partially redundant network of genes that specify photoreceptor cell fate.

Developmental regulation of the cell cycle

The control of metazoan cell proliferation, a problem long the domain of cell culture studies, is now being examined in developing animals. Surprisingly, developmental regulation is mediated at a variety of cell-cycle stages. Highly conserved cell-cycle control mechanisms provide a focus for studying the regulatory processes involved.

Generation and early differentiation of glial cells in the first optic ganglion of Drosophila melanogaster

We have examined the generation and development of glial cells in the first optic ganglion, the lamina, of Drosophila melanogaster. Previous work has shown that the growth of retinal axons into the developing optic lobes induces the terminal cell divisions that generate the lamina monopolar neurons. We investigated whether photoreceptor ingrowth also influences the development of lamina glial cells, using P element enhancer trap lines, genetic mosaics and birthdating analysis. Enhancer trap lines that mark the differentiating lamina glial cells were found to require retinal innervation for expression. In mutants with only a few photoreceptors, only the few glial cells near ingrowing axons expressed the marker. Genetic mosaic analysis indicates that the lamina neurons and glial cells are readily separable, suggesting that these are derived from distinct lineages. Additionally, BrdU pulse-chase experiments showed that the cell divisions that produce lamina glia, unlike those producing lamina neurons, are not spatially or temporally correlated with the retinal axon ingrowth. Finally, in mutants lacking photoreceptors, cell divisions in the glial lineage appeared normal. We conclude that the lamina glial cells derive from a lineage that is distinct from that of the L-neurons, that glia are generated independently of photoreceptor input, and that completion of the terminal glial differentiation program depends, directly or indirectly, on an inductive signal from photoreceptor axons.

The argos gene encodes a diffusible factor that regulates cell fate decisions in the Drosophila eye

The argos gene encodes a protein that is required for viability and that regulates the determination of cells in the Drosophila eye. A developmental analysis of argos mutant eyes indicates that the mystery cells, which are usually nonneuronal, are transformed into extra photoreceptors, and that supernumerary cone cells and pigment cells are also recruited. Clonal analysis indicates that argos acts nonautonomously and can diffuse over the range of several cell diameters. Conceptual translation of the argos gene suggests that it encodes a secreted protein.

Ellipse mutations in the Drosophila homologue of the EGF receptor affect pattern formation, cell division, and cell death in eye imaginal discs

Ellipse alleles are mutations of the EGF-receptor homologue that reduce the number of ommatidia in the eye imaginal disc. Cobalt sulfide staining, expression of hairy and scabrous proteins, and mosaic analysis indicated that Elp mutations affect ommatidial precluster formation in the morphogenetic furrow. BrdU incorporation studies suggest that cells diverted from precluster formation instead enter S-phase after the morphogenetic furrow. Genetic studies suggest that the DER has multiple functions during eye development and that several recessive hypomorphic alleles affect another aspect of DER function that is required after precluster formation. Elp mutations show genetic interactions with the neurogenic mutations Notch and Delta. The small number of ommatidia that differentiate in Elp/Elp are separated more than in wildtype and have been studied to investigate what aspects of ommatidium development are intrinsic to the ommatidium itself. It appears that each developing ommatidium cues the determination of photoreceptors, cone cells, and primary pigment cells, but that the secondary and tertiary pigment cells, and the mechanosensory bristles, can form independently. The normal rotation of ommatidia in the dorsal-ventral axis does not require the presence of the ommatidial array. A short-range signal from a nearby ommatidium is important for mitosis. Cells not close to an ommatidium do not go through mitosis and many die.

A putative Ras GTPase activating protein acts as a negative regulator of signaling by the Sevenless receptor tyrosine kinase

A Drosophila gene with similarity to the mammalian Ras GTPase activating protein has been isolated in screens for mutations that affect eye development. Inactivation of the locus, Gap1, mimics constitutive activation of the Sevenless receptor tyrosine kinase and eliminates the need for a functional Sevenless protein in the R7 cell. Our results suggest that Gap1 acts as a negative regulator of signaling by Sevenless by down-regulating the activity of the Ras1 protein, which has been shown to be a key element in signaling by Sevenless.

The presumptive R7 cell of the developing Drosophila eye receives positional information independent of sevenless, boss and sina

Studies on the development of the R7 photoreceptor in the Drosophila eye thus far have identified three genes that specifically affect this cell: sevenless, boss and sina. In each of these mutants the R7 precursor develops instead as the equatorial cone cell (EQC). We have isolated an enhancer trap line, H214, in which beta-galactosidase is primarily expressed in the R7 cell throughout its development. In mutations of sevenless, boss and sina, expression in H214 is initially reduced although still present in the R7 precursor and persists in the EQC into which it develops. The EQC in wild type never expresses lacZ in H214. This result is in contrast to that seen with other enhancer trap lines that display expression in R7, and indicates that some aspect of R7 differentiation is independent of the genetic pathway(s) involving sevenless, boss and sina.

Localization of DER and the pattern of cell divisions in wild-type and Ellipse eye imaginal discs

The compound eye of Drosophila develops from a uniform layer of epithelial cells in the eye imaginal disc. One intriguing aspect of eye development is the establishment of the correct number and spacing of the photoreceptor clusters which give rise to the mature ommatidia. Ellipse (Elp) has been implicated as playing a role in this process because the Elp dominant gain of function mutation dramatically reduces the number of photoreceptor clusters in the compound eye without affecting the morphology of individual clusters that are formed (Baker and Rubin, 1989). Since Elp represents an allele of the Drosophila EGF receptor (DER) locus, it encodes a protein which is structurally capable of mediating inductive cell-cell interactions. In an effort to better understand the role of the DER locus in ommatidial patterning, we compared the localization of DER protein in eye imaginal discs of wild-type and Elp larvae. The distribution of this receptor is consistent with the notion of its mediating interactions between cells at the initial stages of photoreceptor precluster positioning and differentiation. However, the basis of the Elp gain of function mutation is not ectopic or increased expression of the DER protein. Rather, expression of the Elp form of the EGF receptor homolog in the normal localization leads to changes in the proliferative pattern of cells dividing posterior to the morphogenetic furrow.

Induction in the developing compound eye of Drosophila: multiple mechanisms restrict R7 induction to a single retinal precursor cell

The development of the Drosophila R7 photoreceptor cell is determined by a specific inductive interaction between the R8 photoreceptor cell and a single neighboring precursor cell. This process is mediated by bride of sevenless (boss), a cell-surface bound ligand, and the sevenless (sev) tyrosine kinase receptor. The boss ligand is expressed specifically on the surface of the R8 cell, whereas the sev receptor is expressed on 5 cells contacting the developing R8 cell and other cells not in contact with R8. By altering the spatial and temporal expression of boss, we demonstrate that sev-expressing cells that do not contact R8 can assume an R7 cell fate. By contrast, the sev-expressing precursor cells to the R1-R6 photoreceptor cells that do contact R8 are nonresponsive to the inductive cue. Using the rough and Nspl mutations, we demonstrate that an early commitment to an R1-R6 cell fate blocks the pathway of sev activation in these cells.

Clonal cells from embryonic retinal cell lines express qualitative electrophysiological differences

Cells from the embryonic quail retina were immortalized with the v-mil oncogene and cloned by limiting dilution. Their phenotype was examined using the whole-cell patch clamp method. Three membrane currents, IK(IR), INa and IK, were found at different frequencies within a sample of 170 cells drawn from a large clone. Nearly all combinations of these three markers were found and the frequency of combinations showed that the markers assorted independently. Examination of clones of less than 10 cells showed that heterogeneity originates with a high probability within clones, arguing that chromosomal mutation, for example, is unlikely to account for phenotypic diversity. A possible explanation is that phenotypic differences between cells might reflect the local exchange of instructive signals. If so, then the genes for the three phenotypic markers are controlled independently.

Mechanisms of complex transcriptional regulation: implications for brain development

The large number of transcription factors, their diverse sequence-specific interactions with DNA sites and with other transcription factors, and their ability to be modified in response to a variety of environmental cues and intracellular signals provide combinatorial codes for highly complex and yet highly organized patterns of gene expression likely to underlie the determination of diversity of neuronal phenotypes. Subtle differences in the combinations of transcription factors are likely to have profound consequences for cell phenotype, similar to the mechanism involved in the specification of cell types in yeast (reviewed in Herskowitz, 1989). Although our current understanding of transcriptional regulation in the brain comes largely from phenomenological studies, recent technical progress on two fronts promises a bright future. Homologous recombination technology in embryonic stem cells (reviewed in Capecchi, 1989; Rossant, 1990) allows the disruption of particular genes in transgenic mice and definition of the roles of identified transcription factors in mammalian neurogenesis. A second technological advance, targeted tumorigenesis, has provided neuronal model cell lines (Mellon et al., 1990; reviewed in Cepko, 1988; McKay et al., 1988) that mimic certain neuronal differentiation pathways. These combined genetic, cell biological, and biochemical approaches will greatly facilitate the study of neural development and function.