R8 development in the Drosophila eye: a paradigm for neural selection and differentiation
Benjamin J. Frankfort1,2 and
Graeme Mardon1,2,3,4,5,*
1 Department of Molecular and Human Genetics, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA
2 Department of Pathology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA
3 Department of Ophthalmology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA
4 Department of Neuroscience, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA
5 Program in Developmental Biology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA
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Fig. 1. Dynamic Atonal expression within the morphogenetic furrow. (A) The morphogenetic furrow (MF, blue vertical stripe) traverses the eye imaginal disc from posterior to anterior. Photoreceptors are recruited progressively – ommatidia close to the MF are immature and consist of fewer photoreceptors than do ommatidia located more posteriorly. The first photoreceptor to differentiate with passage of the MF is R8, which then coordinates recruitment of all subsequent photoreceptors. Anterior is towards the left in all panels. (B) Region of box in A. Atonal (Ato) is expressed in a dynamic pattern. Anterior Ato expression is ubiquitous, while posteriorly Ato is resolved to single cells that will become R8s. (C) Schematic of Ato expression. Ato expression occurs in four distinct stages. In this and subsequent figures, gray shading corresponds to the stage of Ato expression. Stage 1 (dark gray): Ato is expressed in a broad band of virtually all nuclei within and just anterior to the morphogenetic furrow. Stage 2 (medium gray): Ato is detected in alternating clusters (intermediate groups) of approximately 10 nuclei each. These intermediate groups are separated by a ‘bridge’ of three or four Ato-expressing cells. Stage 3 (medium gray): Two or three posterior nuclei of the intermediate group migrate apically to form the R8 equivalence group (blue outlines), a group of cells believed to be equipotent to differentiate as the R8 photoreceptor. Stage 4 (light gray): three columns of Ato-expressing nuclei are positioned exactly out of phase with one another to mark the future R8 cell and prefigure the adult hexagonal array. An enhancer located 3' to the ato-coding region controls ato transcription during stage 1 and is not dependent on endogenous ato function, whereas a 5' enhancer is autoregulatory and directs ato transcription during stages 2-4. (D) Lateral view of Ato expression. Ato-expressing nuclei are positioned basally during stages 1 and 2, but migrate toward the apical surface of the imaginal disc during stage 3 and remain apical throughout stage 4.
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Fig. 2. Loss-of-function phenotypes affecting R8 spacing. (A) Loss-of-function N clones result in low levels of Ato expression in stage 1. This represents a failure of ‘proneural enhancement.’ No Ato expression is detected from stage 2-4. (B) Removal of Notch (N) function with a temperature-sensitive mutation results in failure of lateral inhibition. Stage 2 intermediate groups do not resolve and large clusters of well-spaced R8 precursors develop. Stage 1 Ato expression is also reduced at the restrictive temperature. Return to the permissive temperature restores lateral inhibition and single R8 precursors develop. (C) Loss of scabrous (sca) function prevents formation of stage 2 intermediate groups and nuclei at this stage are unpatterned. Notch-mediated inhibition of R8 occurs in stage 2, resulting in single R8 precursors in stage 4, but these cells are too closely spaced and phase relationships are lost. (D) Absence of both N and sca function results in massive overinduction of R8 precursors and nearly all nuclei express Ato during stage 2, owing to a failure of both intermediate group establishment and lateral inhibition. Stage 1 Ato expression is reduced at the restrictive temperature. Upon return to the permissive temperature, single R8 precursors develop, but they are unpatterned. (E) Loss-of-function Epidermal Growth Factor Receptor (EGFR) clones fail to form discrete intermediate groups and have additional Ato-expressing cells in stages 2 and 3. R8 precursors are too closely spaced and lack phase relationships. Note resemblance to sca– phenotype. (F) Removal of both EGFR and sca function leads to a more severe phenotype than either alone. Intermediate groups do not form in stage 2 and many unpatterned Ato-expressing R8 precursors develop in stage 4.
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Fig. 3. R8 selection and spacing. (A) Model for spacing and phase of R8. At least two factors (Scabrous and an unknown EGFR-dependent factor) are secreted from the Atonal-expressing intermediate group to establish a repressive gradient that extends outward in all directions (purple shading). In this model, ato transcription immediately anterior (left) to the intermediate group is repressed (blue inhibitory arrows) even though pre-existing ato transcript and Ato protein are still detectable. The interdigitating areas of ubiquitous Ato (dotted circles) that lie outside the repressive gradient maintain ato transcription and will become the intermediate groups of the next column, thus establishing alternate phase and spacing. (B) Regulation of Ato expression. Stage 1 Ato is controlled by the combined inputs of positively and negatively acting factors. Stage 2 Ato (intermediate groups) marks the onset of Ato autoregulation and induction of Scabrous (Sca), one of the secreted factors that establishes the repressive gradient required for R8 spacing. Sca also potentiates Notch pathway activity during lateral inhibition of stage 2 and 3 Ato. Stage 2 Ato simultaneously induces Rhomboid family members which lead to local EGFR signaling. EGFR signaling is then hypothesized to induce a second factor that contributes to the spacing gradient. This factor is thought to induce Rough (Ro) which represses ato transcription. In stage 3 (R8 equivalence group), Ato induces Sens and is repressed by the actions of both Ro and the Notch pathway. See text for further details. Modified from Baonza et al. (Baonza et al., 2001 ).
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Fig. 4. Genetic control of R8 differentiation. (A) Loss-of-function senseless (sens) clones do not express Ato in stage 4 R8 precursors in most cases and these precursors display proper space and phase relationships. Expression of Ato in stage 1-3 is unaffected. (B) Loss of rough (ro) function prevents resolution of the stage 3 R8 equivalence group into single R8 precursors and two or three R8 precursors express Ato during stage 4 in many ommatidia. Phase relationships and spacing are not affected. Expression of Ato in stage 1 is expanded. (C) Removal of both sens and ro function results in a phenotype that is very similar to ro. (D) Relationships controlling R8 differentiation. sens-mediated repression of ro, a repressor of R8 differentiation, is crucial for R8 differentiation. See text for details.
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Fig. 5. Comparison between Drosophila R8 and SOP development. R8 and SOP development can be divided into four roughly analogous stages: patterning of neural fields, establishment of zones of neural competency (proneural clusters), selection of the presumptive R8/SOP and R8/SOP-mediated recruitment. R8 patterning begins with ubiquitous expression of Ato (light-blue circles) in all retinal cells. The R8 competency zone is determined when ubiquitous Ato is reshaped into clusters of about 10 cells (intermediate groups) via the action of endogenously activated factors. Lateral inhibition represses Ato in all members of the intermediate group except for the selected presumptive R8 (dark blue circles). The differentiating R8 cell recruits neighboring cells, which do not express Ato, to become non-R8 photoreceptors (red circles). These non-R8 photoreceptors have a different function, morphology and axonal projection from R8 cells. As eye development occurs progressively, all stages of R8 specification are visualized simultaneously. By contrast, the future region of an SOP is established by exogenous prepatterning genes (hexagon). All cells within this prepatterned region form a competency zone (proneural cluster) of 15-20 cells and express proneural genes (light-blue circles). Selection occurs when the future SOP expresses proneural genes at a high level and, by lateral inhibition, prevents this enhanced expression in the surrounding cells (dark-blue circles). SOP-mediated recruitment does not occur during external sensory organ (es) development but does occur during chordotonal organ (ch) development. However, the SOP of a chordotonal organ is only sufficient to recruit proneural-expressing cells from within the existing proneural cluster to become SOPs (dark-blue circles). These SOPs then produce equivalent chordotonal organs.
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© The Company of Biologists Ltd 2002
