Cellular interactions in the developing Drosophila eye

Citations Over TimeTop 1% of 1988 papers

Abstract

Cells within developing metazoa are directed to particular pathways of differentiation which lead to the highly ordered tissue constitution of the adult. Two mechanisms, different in principle, that direct developmental decisions are commonly proposed. In lineage mechanisms cells inherit developmental directives and are not influenced by their environment. In position-dependent mechanisms cells assess environmental cues to choose one of many potential developmental pathways. Both mechanisms operate in development and are not mutually exclusive, the final fate of a cell may be determined by a combination of both mechanisms.In the developing eye of Drosophila, as cells choose their terminal cell type, lineage mechanisms do not operate and analysis of this system focuses upon the nature of developmental decisions directed by positional signals. The specific issues that can be addressed are the nature and transmission of the signals, the mechanisms the cells use to receive them, and the molecular events that follow reception and lead to the developmental decisions. The Drosophila eye is particularly advantageous for this as it allows these analyses to be performed at the single cell level. Individual cells can be identified and followed as pattern formation proceeds and genetic screens can be used to identify mutations that alter the developmental pathways of particular cells. Mosaic analyses can indicate in which specific cell(s) a certain gene product is required for normal development. Knowing which cells are required to have a particular gene product and which cells develop inappropriately in its absence indicate a function in the signal or reception side of the cellular communication. Molecular analysis of the gene and localization of the protein give more specific indications of the gene function. By these methods the molecular components of the developmental decision can be systematically dissected.This review describes the recent work, both cellular and molecular, which has begun the investigations into the biochemical components of the developmental decisions. Initial results are promising, suggesting that the positional cues directing cells to their fate are presented by their direct neighbours. The first gene of the system to be analysed in detail, sevenless, is placed on the reception side of the process by mosaic analysis, has a hormone receptor-like amino acid sequence and localization of its protein indicates an interaction with a directly adjacent cell. The findings indicate the potential value of the Drosophila eye for investigation of position-dependent developmental decisions and encourage further efforts.The developmental decisions that lead to the differentiation of the adult retina occur in late larval and early pupal life of the fly. A description of the eye’s structure and constituent cell types is required before the pattern formation that generates it can be adequately discussed.The ommatidium is the simply structured subunit of the compound eye that is reiterated many hundred times (Fig. 1A). It is an assembly of eight photoreceptors and twelve accessory cells in which each cell can be individually identified both by its position and by its type (Dietrich, 1909; Waddington & Perry, 1960; Ready et al. 1976) (Fig. ID). At the core of the ommatidium fie the eight photoreceptors, which can be distinguished into three functionally distinct types: R1–R6, R7 and R8. The rhabdomeres of the outer photoreceptors (Rl–6) extend the depth of the retina and show an asymmetrical arrangement in cross section (Fig. 1B,C). R7 contributes a central rhabdomere in the apical parts of the ommatidium and R8’s rhabdomere occupies the central position in the more basal regions. The asymmetrical phoptoreceptor arrangement occurs in two chiral forms. In the dorsal part of the eye, the photoreceptor pattern is the mirror image of that found in the ventral part. The equator is the boundary of the two types and it runs centrally, dividing the eye into dorsal and ventral halves (Dietrich, 1909). The three classes of photoreceptors have different spectral sensitivities and express different photosensitive pigments (Harris et al. 1976; O’Tousa et al. 1985; Zucker et al. 1985, 1987; Montel et al. 1987; Fryxell & Meyerowitz, 1987). Each of the three types synapses in the optic lobes in different positions; Rl–6 carry short axons which synapse in the lamina ganglion and R7 and R8 project long axons which synapse at different levels in the medulla ganglion. Above the photoreceptors lies the lens system consisting of a fluid-filled pseudocone bordered on top by the corneal lens, laterally by the two primary pigment cells and basally by the four cone cells (Fig. ID). The cone cells are involved in secreting the lens system (Perry, 1968) and can be distinguished into two types, the polar and equatorial pair, which meet centrally separating the anterior and posterior pair (Fig. ID). Surrounding this central group of photoreceptors, cone cells and primary pigment cells is a ring of secondary and tertiary pigment cells which optically insulates the unit. The ommatidial array is hexagonal and, at each alternate vertex of the pattern, a mechanosensory bristle projects from the eye (Fig. 1A,D). The secondary and tertiary pigment cells are shared with neighbouring ommatidia.The colour coding of the cells in Fig. ID and elsewhere is based upon the cell types thought to interact in the developmental programme. This is intended more as an aid to understanding the diagrams since only circumstantial evidence exists to support this cell-type classification.The precise lattice arrangement of the insect retina led Bernard (1937) to propose that it developed by precise cell lineage and for a long time this view remained unchallenged. The first indications that this notion was wrong came from Hotta & Benzer (1970), who noticed that Drosophila gynandromorph clonal patches could cut through individual ommatidia, indicating a nonclonal origin for the ommatidium. This finding was followed up in the Benzer laboratory by Hanson et al. (1972) and Ready et al. (1976) who demonstrated a clear absence of repeatable lineage relationship between cells of an ommatidium. Shelton & Lawrence (1974) reached the same finding with Oncopeltus. These analyses demonstrated that the ommatidium was not clonal, but did not eliminate late cell lineages such as terminal divisions which always generated two photoreceptors or two pigment cells etc. Lawrence & Green (1979), by examining small clonal patches, were able to identify two-cell clones containing both a photoreceptor and a pigment cell. From the absence of cell lineage mechanisms, it is inferred that cells in the developing ommatidia read environmental cues to choose their differentiation pathways.The following description of pattern formation in the developing Drosophila retina, unless otherwise stated, is drawn from Ready et al. (1976), Tomlinson (1985) and Tomlinson & Ready (1987a). Pattern formation in the eye begins during late larval fife in a specialized retinal epithelium, the eye imaginai disc. Ommatidial assembly is initiated in the morphogenetic furrow, a dorsoventral indentation which sweeps anteriorly across the disc with time. Along the length of the furrow, a column of ommatidia begins to assemble, and the furrow subsequently moves anteriorly, resulting in the appearance of one new ommatidial column every 2h (Campos-Ortega & Hofbauer, 1977). As a result, a smoothly graded series of ommatidial development is laid out spatially along the anterior/posterior axis of the disc (Fig. 2A).The eye disc is a monolayer epithelium in which each cell, except for those undergoing division, extends from the apical surface to the basal membrane. Ahead of the furrow, cell division occurs and the nuclei of cells are distributed evenly through the apical/basal axis of the epithelium. As the advancing furrow approaches, cell division ceases and the nuclei of all cells sink and become packed basally in the centre of the furrow. It is at this point that ommatidial assembly begins (Fig. 2A). A group of five nuclei (those of the cells destined to form photoreceptors R2, R3, R4, R5 and R8) rises into the apical region of the epithelium as other cells enter the division cycle. The dividing cells detach basally and round up apically. Division follows and the newly generated cells then extend projections back to the basal lamina and their nuclei migrate basally. This results in a twotier arrangement of the nuclei, with those of R2, R3, R4, R5 and R8 occupying the apical regions and those of all others positioned basally. The five cells form a clearly defined precluster upon which the ommatidium is constructed. Surrounding cells now systematically join the cluster, being incorporated into distinct positions formed by the cells already in the grouping (Fig. 2B). A cell joining the unit first makes a precise set of contacts with neighbouring cluster cells in the apical regions and its nucleus subsequently rises. As a nucleus rises, the contacts made by the cell apically become imposed progressively more basally until they are achieved over the entire depth of the epithelium. As nuclei rise from the basal regions, those already in the apical regions begin to fall and consequently there is a precisely choreographed dance of the nuclei up and down in the epithelium (Fig. 2A).The five-cell unit grows to one of eight cells with the incorporation of those cells destined to become RI, R6 and R7, completing the photoreceptor complement of the ommatidium. The nuclei of R1 and R6 rise first, as those of R2, R5 and R8 fall somewhat, producing a bilaterally symmetrical cluster. The bilateral symmetry results from the paired arrangement of R1 and R6, R2 and R5, and R3 and R4 about R8, with R7 and R8 being bisected by the fine of symmetry (Fig. 2C). The nuclei of R1 and R6 begin to fall as the nucleus of R7 arrives in the apical regions accompanied by the nuclei of the anterior and posterior cone cells. This generates the two-cone cell stage (Fig. 2D). The earlier bilateral symmetry of the unit is now lost as R4 begins to part contact with R8. The equatorial and polar cone cell nuclei now rise as those of R3 and R4 and R7 fall resulting in the fourscone cell stage (Fig. 2E). The adult cellular arrangement of the lens-secreting cone cells overlying the photoreceptors has now been achieved. This is the most advanced developmental stage reached prior to pupation. The developmental programme that builds upon this inner twelve-cell unit and produces the fully differentiated ommatidium during the pupal phase is now being investigated (R. L. Cagan & D. F. Ready, personal communication).The precisely programmed development of the ommatidium, and the strong association between a cell’s fate and its position in the ommatidium suggested that a simple logic of positional cues directs the cells to their fate. However, by morphological analyses alone the five-cell precluster (R2, R3, R4, R5 and R8), the foundation unit of the ommatidium, was the earliest event detectable and the developmental sequence that establishes it could not be ascertained. However, antibody analyses were able to break it down into its component differentiation steps.Antibodies raised against many different epitopes stain developing neurones in the eye imaginai disc; MAb 22C10 (Fujita et al. 1982), A-HRP (Jan & Jan, 1982), Sox-2 (Goodman et al. 1984), Drosp 302 (Lebovitz & Ready, 1986). The antibodies recognize the developing photoreceptors and highlight the ommatidial array beginning in the morphogenetic furrow and extending to the posterior of the disc. Using neuronal antibodies as differentiation markers Tomlinson & Ready (1987a) found that the eight photoreceptors differentiate in a precise sequence. R8 is first, followed by the pair R2 and R5, then the pair R3 and R4, followed by the pair R1 and R6 and lastly R7 (Fig. 2F–J). Several antibodies have been used to confirm this developmental sequence which correlates well with the morphological analyses and subdivides the precluster into three stages of maturation – R8, R2 and R5, and R3 and R4. Combining the antibody analyses with the morphological ones gives the differentiation sequence of the inner twelvecell unit of the ommatidium shown in Fig. 2K.The absence of cell lineage in the developing Drosophila retina led Ready et al. (1976) to propose that the eye grew like a crystal with the organization of cells along the morphogenetic furrow being used as a template. From their analysis of the Oncopeltus retina, Shelton & Lawrence (1974) envisaged the ommatidium as the unit crystal with a cell’s position within the cluster being the key factor directing its fate. Lebovitz & Ready (1986) demonstrated the ability of dissected and transplanted Drosophila retinal tissue from ahead of the morphogenetic furrow to differentiate ommatidia and concluded from this and other evidence that the ommatidia are essentially selfassembling units. There are many indications of the autonomy of assembling ommatidia. A good example is the mutant Ellipse in which individual ommatidia can be observed differentiating, apparently normally, well isolated from others (N. E. Baker, personal communication; Fig. 5B).Tomlinson & Ready (1987a) inferred that the photoreceptor differentiation sequence (R8, R2 and R5, R3 and R4, R1 and R6, R7) directly the sequence of these cells. The was that a cell differentiated before it been directed to its differentiation a of the photoreceptors are clearly of at three distinct types (R8, Rl–6 and R7, and at four types (R8, and R7, Fig. was about the differentiation sequence of the photoreceptors was that it in of cell the R8 type is followed by the type, then R3 and R4 and R1 and R6 and lastly the R7 of the cells are determined as but on of the cluster indicating that cues for these cells are presented on both of the unit. The autonomy of the assembling ommatidia indicates that the positional cues directing the cells are to the ommatidia the ommatidium on both and in sequence of cell type positional cues to of the suggested that cells read the of the cells they contact in to their fate & Ready, This an cell a precise set of contacts with cells of the cluster and the combination of cell types it then contacts an to its fate. a cell has been directed to its fate it then express to its type that to the of cells that it subsequently of cell contact directing cell fate makes certain an cell’s contacts alter its fate. the Drosophila retina is to such as and cellular which have been used to such in the & and & and many other mutations in involved in the or reception of the cells to Ommatidial development in such up to the point the particular gene product is required but then from the cellular sevenless, which is with at length is such a and in a of eye Tomlinson & D. F. Ready, was identified as (Fig. In ommatidia, the differentiation of R8 followed by R2 and R5 normal but the of R3 and R4 to the unit not occur and the ommatidia develop in an and of this gene has shown it a The gene product is only required in R2 and R5 for normal ommatidial development but it is R3 and R4 that indicating an on the signal reception side of the E. & can be used to the involved in the may be identified by antibody screens and gene could be by the use of & (Fig. is an identified in laboratory in a for by its Each ommatidium of the eye the R7 photoreceptor (Harris et al. 1976; et al. (Fig. ommatidial development in is to type with a cell occupying the cluster position which generates the R7 The cell in the R7 position to the R7 for many but then to the photoreceptor differentiation and the equatorial cone cell, one of the four cells involved in lens & Ready, (Fig. is a the of a cell destined to one particular differentiation to that of analysis was used to the cell in the R7 position was being by mutant environmental signals, or the cell was positional cues and was not (Harris et al. 1976; et al. Tomlinson & Ready, In analysis of many hundred ommatidia mosaic for and cells a R7 cell was observed and a mutant cell in the R7 position in the developing ommatidium could not be by R7 cells could form of the of other cells (Fig. These analyses that the gene product was involved in the reception of the cues directing the R7 cell type and its gene has been isolated et al. et al. and the sequence it to be to hormone such as the the and in that it is a protein with an et al. 1987). However, its is amino and the protein may have two & et al. The receptor-like structure of the protein led et al. to propose that the protein was involved in that direct the fate of the R7 cell. suggested that of a to the of the and a By a single amino acid within the of the & demonstrated of gene with the signal the the gene product is involved in reception of positional cues directing the fate of the R7 cell, then certain can be for the time and of of the ommatidial cells are prior to then many of are of R7, and cells other the R7 cell express the the protein be in the R7 prior to that cell of have been raised to the protein et al. Tomlinson et al. (Fig. and it is found in the R7 prior to differentiation and in many other with a for the protein in positional cues for the R7 The two of protein localization in specific et al. (1987a) used clonal antibodies generated against a protein and only in the apical the and in all cells of Tomlinson et al. used raised against and in many cells of the developing ommatidia but not in R8, R2 and found in the apical the of the in certain positions and times at the of the and in The at the of the was in that it only in cells that the protein and R8 R4 and was at the position these cells R8 but not directly or they other cells (Fig. This led to the that a for the protein is on the R8 cell. a on the of has been identified which results in an eye to with each ommatidium the R7 cell (R. & L. personal analysis of ommatidia mosaic for and tissue has that a normal ommatidium is generated only the R8 cell is type and a ommatidium results the R8 cell is of the of R7 or that of other cells. the gene product is required in the R8 cell for the ommatidium to an R7 cell and is with the that the R8 cell a for the the localization of the protein to the contact with R8 indicates then of the protein at these It is that this occurs in R3 and R4 as well as R7 since of gene function has upon R3 and R4. The of the protein is that is it that cells their fate they down the and may a of for cells to be directed to their fate. The by R3 and R4 of protein may indicate that cells to assess their fate express a of one of which can be sevenless, and the combination of those fate. This that other show to is a for understanding the of certain hormone of structure to the protein can lead to of cells and are directed understanding an to the of signal by such in a Drosophila not only strong genetic support for such but the highly defined development programme of the eye allows the of such a to be analysed at the of the single the pattern formation have with the assembly of individual ommatidia, but the developing retina as a has which be from this Drosophila retina has two chiral of ommatidia. in the ventral are the mirror image of those in the dorsal and the two meet along the equator that the eye (Dietrich, 1909). In the morphogenetic furrow, ommatidia are symmetrical which all directly As development proceeds they begin to those on one side of the equator in the to those on the other until they to point directly at each other (Fig. As the occur become incorporated into the ommatidia which with the side of the equator they are on and the in which they & Ready, of more insect are are and equator However, in these ommatidia the basal projections from the cone cells a on the ommatidia and two chiral one form in the dorsal and the other in the ventral F. Ready, personal it is that in the Drosophila retina the the on the ommatidia, the of the ommatidia and the in which they from an which the dorsal part of the developing retina from the ventral part. can be to this there exists evidence as to the the length of the morphogenetic furrow ommatidial development is initiated at evidence exists to the which establishes this However, & which the apical of cells in the region of the morphogenetic furrow, has a & Ready, In the centre of the furrow, long of cells to the axis of the furrow can be (Fig. The at along their length and as development proceeds it as of cells in these regions detach from the and then begin to As occurs five of the cells (those destined to be R2, R3, R4, R5 and R8) a precise set of contacts with each other and to form the precluster other cells are from the unit. many other cells are involved is not at one cell can always be with the in these early of early with the antibody four cells R4, and cell, within the developing cluster, indicating that cells may be in the formation of the precluster et al. 1987). The fate of the cells is The of development at along the furrow and the interaction of cells in these regions to form the precluster to the mechanisms in other between cells of the in an of along the cellular et al. formation in insect & positional mechanisms are thought to a differentiate in the and cells in these regions then to become the mechanisms are envisaged for insect bristle is from and biochemical analyses of Drosophila retinal development that the positional cues directing cells to their fate can be directly presented by adjacent cells. the are on the neighbouring or are from these cells is not Cells to be directed by the combination of cell types they of R7 is the cell fate in the developing ommatidium. Two are in is required in the R7 and is required in R8. The that the formation of R7 be upon gene in cells other R7 and are many of developmental such as in which direct may but in only a has this been precisely The good The is initiated by direct contact as is maturation and through direct cell are required from cells for maturation et al. of the Drosophila gene have it a protein involved in cellular cell fate in the et al. 1985; & & 1987). As molecular and biochemical analyses of in many are the to which they with the mechanisms of Drosophila ommatidial cells become like to Ready for and for being there and for results on prior to for and and of the and Benzer for on the

Related Papers

• → The Hippo pathway effector Yki downregulates Wg signaling to promote retinal differentiation in the Drosophila eye(2015)40 cited
• → Spatiotemporal patterning of polyamines in Drosophila development(2015)10 cited
• → Fast and efficient egg collection and antibody staining from large numbers of Drosophila strains(1997)15 cited
• → Analyzing the role of CtBP in Drosophila eye development(2008)
• → The Hippo pathway effector Yki downregulates Wg signaling to promote retinal differentiation in the Drosophila eye(2015)