Developing compound eye in lozenge mutants of Drosophila: lozenge expression in the R7 equivalence group

 The lozenge locus is genetically complex, containing two functionally distinct units, cistrons A and B, that influence the structure of the compound eye. Extreme mutations of either cistron produce adult phenotypes that share similarities and that have striking differences. We have analyzed the expression of several developmentally important eye genes including boss, scabrous, rhomboid, seven-up, and Bar in lozenge mutant backgrounds representing both cistrons. This analysis follows the progressive recruitment of photoreceptor neurons during eye development and has confirmed that the initial development of photoreceptors is normal up to the five cell precluster stage (R8, R2/5 and R3/4). However, when lozenge is mutant, further eye development is perturbed. As cells R1, R6 and R7 are recruited, patterns of gene expression for seven-up and Bar become abnormal. We have also characterized the expression of two different enhancer trap alleles of lozenge. The lozenge product(s) appear to be first expressed in the eye disc in undifferentiated cells shortly after the five cell precluster forms. Then, as distinct cells are recruited to a fate, lozenge expression persists and is refined in those cells. Our data suggests that lozenge functions in cone cells and pigment cells as well as in specific glia. With respect to photoreceptor neurons, lozenge biases the developmental potential of cells R1, R6 and R7, by directly influencing the expression of genes important for establishing cell fate. Received: 26 July 1996 / Accepted: 6 January 1997     

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.     

Specification of cell fate in the developing eye of Drosophila.

Determination of cell fate in the developing eye of Drosophila depends on a precise sequence of cellular interactions which generate the stereotypic array of ommatidia. In the eye imaginal disc, an initially unpatterned epithelial sheath of cells, the first step in this process may be the specification of R8 photoreceptor cells at regular intervals. Genes such as Notch and scabrous, known to be involved in bristle development, also participate in this process, suggesting that the specification of ommatidial founder cells and the formation of sensory organs in the adult epidermis may involve a similar mechanism, that of lateral inhibition. The subsequent steps of ommatidial assembly, following R8 assignment, involve a different mechanism: Undetermined cells read their position based on the contacts they make with neighbors that have already begun to differentiate. The development of the R7 photoreceptor cell, one of the eight photoreceptor cells in the ommatidium, is best understood. An important role seems to be played by sevenless, a receptor tyrosine kinase on the surface of the R7 precursor. It transmits the positional information--most likely encoded by the boss protein on the neighboring R8 cell membrane--into the cell via its tyrosine kinase, which activates a signal transduction cascade. Constitutive activation of the sevenless kinase by overexpression of an N-terminally truncated form results in the diversion of other ommatidial cells into the R7 pathway suggesting that activation of the sevenless signalling pathway is sufficient to specify R7 development. Genetic dissection of this pathway should therefore identify components of a signalling cascade activated by a tyrosine kinase.     

Tiling mechanisms of the Drosophila compound eye through geometrical tessellation

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Cell-cycle regulation and cell-type specification in the developing Drosophila compound eye

During nervous system development stem cell daughters must exit the proliferative cycle to adopt specific neural and glial fates and they must do so in the correct positions. Cell proliferation in the central nervous system occurs in neuroepithelia such as the neural retina and the ventricular zones. As cells are assigned specific fates they migrate out of the plane of the epithelium to form higher layers. Recent evidence from the Drosophila compound eye suggests that a novel mode of Ras pathway regulation may be crucial in both cell-cycle exit and neural patterning: "MAP Kinase cytoplasmic hold".     

The molecular basis of pattern formation in the developing compound eye of Drosophila

Throughout metazoan development cells select pathways of specialization that lead to the differentiation of specific cell types. Differential gene activation converts initially homogeneous populations of cells into spatial arrangements of diverse cell types. As discussed in other articles in this issue, the signals specifying divergent pathways can be encoded in a cell's lineage, its environment, or a combination of both. This article reviews recent analyses of the developing Drosophila compound eye which have focussed upon the mechanisms by which cells assess environmental information in order to determine their fate. More specifically, it examines the molecular mechanisms used by cells to communicate signals which instruct the developmental pathways of other cells.     

An overexpression screen in Drosophila for genes that restrict growth or cell-cycle progression in the developing eye

We screened for genes that, when overexpressed in the proliferating cells of the eye imaginal disc, result in a reduction in the size of the adult eye. After crossing the collection of 2296 EP lines to the ey-GAL4 driver, we identified 46 lines, corresponding to insertions in 32 different loci, that elicited a small eye phenotype. These lines were classified further by testing for an effect in postmitotic cells using the sev-GAL4 driver, by testing for an effect in the wing using en-GAL4, and by testing for the ability of overexpression of cycE to rescue the small eye phenotype. EP lines identified in the screen encompass known regulators of eye development including hh and dpp, known genes that have not been studied previously with respect to eye development, as well as 19 novel ORFs. Lines with insertions near INCENP, elB, and CG11518 were characterized in more detail with respect to changes in growth, cell-cycle phasing, and doubling times that were elicited by overexpression. RNAi-induced phenotypes were also analyzed in SL2 cells. Thus overexpression screens can be combined with RNAi experiments to identify and characterize new regulators of growth and cell proliferation.     

act up controls actin polymerization to alter cell shape and restrict Hedgehog signaling in the Drosophila eye disc

Cells in the morphogenetic furrow of the Drosophila eye disc undergo a striking shape change immediately prior to their neuronal differentiation. We have isolated mutations in a novel gene, act up (acu), that is required for this shape change. acu encodes a homolog of yeast cyclase-associated protein, which sequesters monomeric actin; we show that acu is required to prevent actin filament polymerization in the eye disc. In contrast, profilin promotes actin filament polymerization, acting epistatically to acu. However, both acu and profilin are required to prevent premature Hedgehog-induced photoreceptor differentiation ahead of the morphogenetic furrow. These findings suggest that dynamic changes in actin filaments alter cell shape to control the movement of signals that coordinate a wave of differentiation.     

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. © 1996 John Wiley & Sons, Inc.     

senseless repression of rough is required for R8 photoreceptor differentiation in the developing Drosophila eye.

An outstanding model to study how neurons differentiate from among a field of equipotent undifferentiated cells is the process of R8 photoreceptor differentiation during Drosophila eye development. We show that in senseless mutant tissue, R8 differentiation fails and the presumptive R8 cell adopts the R2/R5 fate. We identify senseless repression of rough in R8 as an essential mechanism of R8 cell fate determination and demonstrate that misexpression of senseless in non-R8 photoreceptors results in repression of rough and induction of the R8 fate. Surprisingly, there is no loss of ommatidial clusters in senseless mutant tissue and all outer photoreceptor subtypes can be recruited, suggesting that other photoreceptors can substitute for R8 to initiate recruitment and that R8-specific signaling is not required for outer photoreceptor subtype assignment. A genetic model of R8 differentiation is presented.     

Migration and function of glia in the developing Drosophila eye.

Although glial cells have been implicated widely in the formation of axon tracts in both insects and vertebrates, their specific function appears to be context-dependent, ranging from providing essential guidance cues to playing a merely facilitory role. Here we examine the role of the retinal basal glia (RBG) in photoreceptor axon guidance in Drosophila. The RBG originate in the optic stalk and have been thought to migrate into the eye disc along photoreceptor axons, thus precluding any role in axon guidance. Here we show the following. (1) The RBG can, in fact, migrate into the eye disc even in the absence of photoreceptor axons in the optic stalk; they also migrate to ectopic patches of differentiating photoreceptors without axons providing a continuous physical substratum. This suggests that glial cells are attracted into the eye disc not through haptotaxis along established axons, but through another mechanism, possibly chemotaxis. (2) If no glial cells are present in the eye disc, photoreceptor axons are able to grow and direct their growth posteriorly as in wild type, but are unable to enter the optic stalk. This indicates that the RBG have a crucial role in axon guidance, but not in axonal outgrowth per se. (3) A few glia close to the entry of the optic stalk suffice to guide the axons into the stalk, suggesting that glia instruct axons by local interaction.     

Cell-fate determination in the developing Drosophila eye: role of the rough gene.

The homeobox-gene rough is required in photoreceptor cells R2 and R5 for normal ommatidial assembly in the developing Drosophila eye. We have used several cell-type-specific markers and double mutant combinations to analyze cell-fate determination in rough. We show that the cells that would normally become R2 and/or R5 express a marker, a lacZ insertion in the seven-up (svp) gene, which is indicative of the R1/3/4/6 cell fate. In addition, the analysis of mitotically induced svp,ro double mutant clones in the eye indicates that in rough all outer photoreceptors are under the genetic control of the svp gene. These results show that, in the absence of rough function, R2 and R5 fail to be correctly determined and appear to be transformed into cells of the R3/4/1/6 subtype. This transformation and the subsequent developmental defects do not preclude the recruitment of R7 cells. However, the presence of ommatidia containing more than one R7 and/or R8 cell in rough implies a complex network of cellular interactions underlying cell-fate determination in the Drosophila retina.