Building an ommatidium one cell at a time

Abstract

Since the discovery of a single white-eyed male in a population of red eyed flies over 100 years ago (Morgan, 1910), the compound eye of the fruit fly, Drosophila melanogaster, has been a favorite experimental system for identifying genes that regulate various aspects of development. For example, a fair amount of what we know today about enzymatic pathways and vesicular transport is due to the discovery and subsequent characterization of eye color mutants such as white. Likewise, our present day understanding of organogenesis has been aided considerably by studies of mutations, such as eyeless, that either reduce or eliminate the compound eyes. But by far the phenotype that has provided levers into the greatest number of experimental fields has been the humble "rough" eye. The fly eye is composed of several hundred unit-eyes that are also called ommatidia. These unit eyes are packed into a hexagonal array of remarkable precision. The structure of the eye is so precise that it has been compared with that of a crystal (Ready et al., 1976). Even the slightest perturbations to the structure of the ommatidium can be visually detected by light or electron microscopy. The cause for this is two-fold: (1) any defect that affects the hexagonal geometry of a single ommatidium can and will disrupt the positioning of surrounding unit eyes thereby propagating structural flaws and (2) disruptions in genes that govern the development of even a single cell within an ommatidium will affect all unit eyes. In both cases, the effect is the visual magnification of even the smallest imperfection. Studies of rough eye mutants have provided key insights into the areas of cell fate specification, lateral inhibition, signal transduction, transcription factor networks, planar cell polarity, cell proliferation, and programmed cell death just to name a few. This review will attempt to summarize the key steps that are required to assemble each ommatidium.

Figure 1. Structure of the adult compound eye

(A) Scanning electron micrograph of an adult wild type Drosophila compound eye. (B) Light microscope section of an adult wild type compound eye. Note the change in chirality of the ommatidia at the equator. (C) A high magnification view of a single ommatidium. The photoreceptors are numbered according to the convention established by Dietrich, 1909. Note that the photoreceptors are arranged in an orientation that resembles an assymetrical trapezoid. Anterior is to the right on all images.

Figure 2. Cellular composition and chirality of the Drosophila ommatidium

(A) An illustration describing the cellular composition of a single ommatidium. (B) An illustration describing the four-fold symmetry that exists within the two compound eyes of a single adult fly. Note that the photoreceptors are arranged in an orientation that resembles an asymmetrical trapezoid. Anterior is to the right in all illustrations.

Figure 3. The morphogenetic furrow and the assembly of the ommatidium

(A–E) Confocal images of developing wild type eye eye discs. The adjacent antennal disc has been removed in each images. red = F-actin, green = ELAV. Note that the morphogenetic furrow initiates at the posterior margin and traverses the disc towards the anterior edge of the epithelium. (F) An illustration depicting the developmental history of an ommatidium. Note that the eight-cell mature cluster has two-fold symmetry. This symmetry will be broken as the ommatidium rotates and adopts the asymmetric trapezoidal orientation of the adult retina. Anterior is to the right in all images and illustrations.

Figure 4. Order of cone, pigment and bristle cell recruitment in the pupal retina

(A–F) An illustration depicting the order in which all non-neuronal cells are recruited into the ommatidium. The photoreceptor clusters are marked in pink and sit below the quartet of cone cells in each panel. See figure 2 for a description of each cell type. Anterior is to the right in each illustration.

Figure 5. Proneural selection of the R8 cell requires refinement of the atonal expression pattern

(A) An illustration documenting the steps in the refinement of the ato expression pattern. Note that Notch is required for the transition from the intermediate group to the R8 equivalence group and that ro refines ato expression with the equivalence group. (B) Description of the genotype of the intermediate group, equivalence group, R8 and R2/5 pair photoreceptor pair. (C) Regulatory circuit between the ro, sens and ato genes within the equivalence group. Anterior is to the right in each illustration.

Figure 6. The EGF Receptor and a combinatorial code of transcription factors govern cell fate decisions

(A) A model of how the Egfr pathway regulates cell fate specification within the ommatidium. The original source of the ligand is the R8. It is secreted and received by neighboring cells. As each cell is recruited into the ommatidium it in turn will generate a new Spi signal and the process repeats itself. At the same time an inhibitory ligand, Argos, is also generated. In situations where Spi and Argos are present at high levels the pathway promotes ommatidial assembly. In cells where Argos found at higher levels than Spi then the pathway blocks differentiation. The downstream effectors of the Egfr pathway (Pnt and Yan) are also differentially expressed and these aid in the promotion and inhibition of differentiation respectively. (B) Schematics depicting the expression patterns of several genes that govern the fate of the outer photoreceptors. Only the photoreceptors and cone cells are depicted. Anterior is to the right in each illustration.

Figure 7. The R7 fails to form in sevenless mutant ommatidia

(A) Light microscope section of an adult wild type retina. Note the presence of seven photoreceptors in each ommatidium. The R8 cell sits below the R7. (B) Light microscope section of an adult sevenless retina. Note that in nearly all ommatidia only six photoreceptors are present. The R7 has been converted into an equatorial cone cell. The R8 is not seen within this plane of field. Note that a single ommatidium has an intact R7 cell (upper left hand corner). Anterior is to the right in both images

Figure 8. Specification of the R7 neuron by the Sevenless signal transduction pathway

(A) An illustration depicting the core of the Sevenless pathway. The small dark blue spheres represent phosphorylation events. (B) Schematics depicting the expression patterns of several genes that control the fate of the R7 photoreceptor. Only the photoreceptors and cone cells are depicted. Anterior is to the right panel B.

Figure 9. Molecular mechanisms of cone and primary pigment cell development

(A) A depiction of the regulatory inputs into the sparkling enhancer of the DPax2 gene. Several transcription factors such as Lz, Su(H), Pnt and Yan are known to bind and are required for proper expression of DPax2 in the cone cells. A recent molecular analysis of the sparkling enhancer has identified additional sequences that are important for the proper functioning of the enhancer (Swanson et al., 2010). The identity of the predicted DNA binding proteins (X, Y, Z) that interact with these sites are still unknown. (B) The source of the ligands for both EGF and Notch receptors are the developing photoreceptors. (C) The Notch pathway controls the fate of the primary pigment cells. The source of the activating ligand is the lens secreting cone cells. Anterior is to the right in panels B and C.

Figure 10. Schematic of phenotypes associated with loss-of-function mutations in genes that govern ommatidial assembly

(A–O) Depictions of the mutant phenotypes associated with each gene that has been discussed within this review. Note that although the R8 cell is not shown in most panels it is present in all mutants save ato in panel B. Anterior is to the right in each illustration.