Regional modulation of a stochastically expressed factor determines photoreceptor subtypes in the Drosophila retina

Abstract

Stochastic mechanisms are sometimes utilized to diversify cell fates, especially in nervous systems. In the Drosophila retina, stochastic expression of the PAS-bHLH transcription factor Spineless (Ss) controls photoreceptor subtype choice. In one randomly distributed subset of R7 photoreceptors, Ss activates Rhodopsin4 (Rh4) and represses Rhodopsin3 (Rh3); counterparts lacking Ss express Rh3 and repress Rh4. In the dorsal third region of the retina, the Iroquois Complex transcription factors induce Rh3 in Rh4-expressing R7s. Here, we show that Ss levels are controlled in a binary on/off manner throughout the retina yet are attenuated in the dorsal third region to allow Rh3 coexpression with Rh4. Whereas the sensitivity of rh3 repression to differences in Ss levels generates stochastic and regionalized patterns, the robustness of rh4 activation ensures its stochastic expression throughout the retina. Our findings show how stochastic and regional inputs are integrated to control photoreceptor subtype specification in the Drosophila retina.

Figure 1. Rhodopsin expression in the fly eye

For A-C, top left indicates rhabdomeres (membranous structures containing Rh proteins) within an ommatidium. Top right indicates cross-sections in the R7 (top) and R8 (bottom) layers. Gray indicates cell bodies and nuclei. White circles with black outlines indicate outer PR rhabdomeres. Central, colored rhabdomeres indicate R7 (top) or R8 (bottom). Below, the regulatory network controlling Rh expression in R7 (top) or R8 (bottom).

1. A.

   pale: Rh3 (blue) is expressed in pR7s and Rh5 (purple) is expressed in pR8s. In the absence of Ss in pR7s, Sal represses Dve and acts with Otd to induce Rh3 expression. Rh4 expression is not activated. In the absence of Ss, a signal is de-repressed which upregulates expression of the growth regulator Melted (Melt) in R8s. Melt represses the tumor suppressor Warts (Wts) inducing expression of Rh5 and repression of Rh6.

2. B.

   yellow: Rh4 (red) is expressed in yR7s and Rh6 (green) is expressed in yR8s. In yR7s, Ss activates Rh4 and Dve, which represses Rh3. Ss also represses the signal to R8. Melt is not expressed in the absence of the signal, allowing for Wts expression inducing expression of Rh6 and repression of Rh5.

3. C.

   dorsal third yellow: Rh3 (blue) and Rh4 (red) are expressed in yR7s and Rh6 (green) is expressed in yR8s. In yR7s, Ss activates Rh4 and Dve. In the dorsal third region, Ss levels are reduced to allow for IroC-mediated activation of Rh3. Ss also represses the signal to R8. Melt is not expressed in the absence of the signal, allowing for Wts expression inducing expression of Rh6 and repression of Rh5.

For D-F, dorsal is up and ventral is down.

1. D.

   In the main part of the retina, Rh3 and Rh4 are expressed in stochastic and exclusive subsets of R7s. Main part = below white dashed line; dorsal third region = above the white dashed line.

2. E.

   Higher magnification view of dorsal third region. In the dorsal third region, Rh3 is expressed in all R7s including Rh4-expressing yR7s. Top (Rh3 and Rh4) and bottom (Rh3 alone).

3. F.

   Rh5 and Rh6 are expressed in stochastic and exclusive subsets of R8s.

Figure 2. Tgo is required for Ss-mediated regulation of Rhs

1. A.

   Schematic of the tgo gene locus. The blue triangle indicates the P-element containing the hobo transposon used to generate the tgo^del25 and tgo^del6molecular null alleles.

2. B.

   Schematic of the Tgo protein with mutant allele annotation.

3. C.

   Rh3 and Rh4 are expressed in random subsets of R7s in wild type animals.

4. D.

   Rh3 is expressed in all R7s in ss null mutants.

5. E.

   Rh3 is expressed in all R7s in tgo^del6 null mutants.

6. F.

   Rh3 is expressed in all R7s when tgo is knocked down by RNAi.

7. G.

   Rh5 and Rh6 are expressed in random subsets of R8s in wild type animals.

8. H.

   Rh5 frequency increases (Rh6 decreases) in ss null mutants.

9. I.

   Rh5 frequency increases (Rh6 decreases) in tgo^del6 null mutants.

For J-N, GFP- indicates tgo^del6 mutant tissue; GFP+ indicates wild type tissue. Left, stain including GFP; Right, stain without GFP.

1. J.

   Rh3 is expressed in all R7s in tgo^del6 clones. Rh3 and Rh4 are expressed in random subsets of R7s in wild type tissue.

2. K.

   Ss nuclear expression is lost in tgo^del6 clones. Ss is expressed in random subsets of R7s in wild type tissue. White circles indicate Ss+ R7s; gray circles indicate Ss- R7s.

3. L.

   Dve expression is lost only in R7s in tgo^del6 clones. Dve is expressed in random subsets of R7s in wild type tissue. Dve expression occurs in outer PRs in tgo^del6and wild type tissue. White circles indicate Dve+ R7s; gray circles indicate Dve-R7s.

4. M.

   Ectopic expression of Ss induces Rh4 in R7s and outer PRs in wild type tissue. Rh3 is expressed in all R7s in tgo^del6 clones with ectopic expression of Ss.

5. N.

   Strong nuclear expression of Ss is observed upon ectopic expression in all PRs in wild type tissue. Weak nuclear expression of Ss is observed upon ectopic expression in all PRs in tgo^del6 clones.

6. O.

   tgo^prom>GFP is expressed in all PRs of the retina including all R7s, R8s, and outer PRs. See also Figure S1.

Figure 3. Ss levels are regionally modulated

For A-H, DT=dorsal third, DE=dorsal equatorial, VE=ventral equatorial, VT=ventral third. For B, D, F, and H, the bimodal distribution indicates the On/Off nature of Ss expression. The shift for the DT R7s (Figure 3B) shows the reduced levels of Ss expression in the On State in this region

1. Ss expression is lower in DT R7s.

2. Quantification of Ss expression in DT R7s.

3. Ss expression in DE R7s.

4. Quantification of Ss expression in DE R7s.

5. Ss expression in VE R7s.

6. Quantification of Ss expression in VE R7s.

7. Ss expression in VT R7s.

8. Quantification of Ss expression in VT R7s.

9. Ss levels are similar in IroC mutant and wild type clones. GFP- indicates IroC mutant tissue; GFP+ indicates wild type tissue. Top, Ss and GFP; Bottom, Ss alone. White circles indicate high Ss expression in yR7s; gray circles indicate no Ss expression in pR7s.

10. Quantification of Ss levels in IroC mutant and wild type clones. IroC mutant yR7s (gray) express Ss at similar levels to wild type yR7s (red).

Figure 4. Rh3 and Rh4 are differentially responsive to Ss/Tgo activity levels

For A-G, DT=dorsal third, DE=dorsal equatorial, VE=ventral equatorial, VT=ventral third. For A-F, top, Rh3 and Rh4; middle, Rh3 alone; bottom, Rh4 alone. Green lines mark the regional boundary of Rh3 expression in yR7s. Dotted yellow lines mark the regional boundary of the normal frequency of Rh4 expression in yR7s.

1. Ectopic expression of Ss (rh4>ss) represses Rh3 in dorsal third yR7s. Rh3 and Rh4 are expressed in exclusive subsets of R7s in all regions.

2. In wild type animals, Rh3 and Rh4 are expressed in exclusive subsets of R7s in the DE, VE, and VT regions. The DT region is composed of R7s that express Rh3 alone or co-express Rh3 with Rh4.

3. In tgo⁶ mutants, Rh3 expression in yR7s expands to the DE region. Rh3 and Rh4 are expressed in exclusive subsets of R7s in the VE and VT regions. The DT and DE regions are composed of R7s that express Rh3 alone or co-express Rh3 with Rh4.

4. In tgo⁵ mutants, Rh3 expression in yR7s expands to the entire retina. These retinas are composed of R7s that express Rh3 alone or co-express Rh3 with Rh4.

5. In ss¹¹⁶⁵ mutants, Rh3 expression in yR7s expands to the entire retina. These retinas are composed of R7s that express Rh3 alone or co-express Rh3 with Rh4. The frequency of Rh4 expression is slightly reduced in the DT.

6. In tgo^del6 mutants, Rh4 is lost and Rh3 is expressed in all R7s throughout the retina.

7. Quantification of the series of ss and tgo alleles. Data is presented in order of decreasing Ss/Tgo activity (i.e. increasing phenotypic severity). The six main phenotypic classes are separated by dashed lines.

Figure 5. Ss/Tgo with reduced activity can activate Rh4 but not repress Rh3

1. A.

   Schematic of the Ss protein. The premature stop causing protein truncation in the ss^d116.4 allele is annotated.

2. B.

   Table summarizing Ss protein domain analysis. −> Rh4 = Rh4 activation; −| Rh3 = Rh3 repression; Tgo nuclear local = Tgo localization. (−) indicates no effect. (+) indicates an effect. For the Tgo nuclear localization assay, (++) indicates strong nuclear localization whereas (+) indicates weak nuclear localization.

For C-H, examples of each type of phenotype are shown. For Rh3/Rh4 expression and Tgo nuclear localization, image data for all constructs are shown in Figures S2K–BB. For C-E, examples of effects on Rh3/Rh4 expression. Left, Rh3 and Rh4 expression; middle, Rh3 alone; right, Rh4 alone. Expression was assessed in the main part of the retina (excluding the dorsal third).

1. C.

   Activation of Rh4 with repression of Rh3.

2. D.

   Activation of Rh4 without repression of Rh3. Yellow circles indicate examples of R7s that co-express Rh3 and Rh4.

3. E.

   No effect

4. F.

   For F-H, examples of effects on Tgo nuclear localization.

5. F.

   Strong nuclear localization of Tgo.

6. G.

   Weak nuclear localization of Tgo.

7. H.

   No effect.

See also Figure S2.

Figure 6. Regional rh3 expression is sensitive to activating and repressing inputs

1. A.

   Dve levels in yR7s are decreased in tgo⁵ mutant clones. GFP- indicates tgo⁵mutant tissue; GFP+ indicates wild type tissue. Left, Dve and GFP; Right, Dve alone. White circles indicate high Dve expression in wild type yR7s; light gray circles indicate low Dve expression in tgo⁵ mutant yR7s; dark gray circles indicate no Dve expression in wild type and tgo⁵ mutant pR7s

2. B.

   Quantification of Dve levels in tgo⁵ mutant clones and wild type tissue. Wild type R7s (green) express Dve at higher levels than tgo⁵ mutant R7s (black).

3. C.

   Schematic of rh3 promoter that recapitulates Rh3 protein expression. Orange = K₅₀, Purple = IroC site, Green = RCSI, Yellow = TATA box. The RCSI is a conserved element found in all rh promoters that is required for expression.

For D, gray circles indicate DT yR7s that have lost expression of rh3^prom>GFP. For E-H, white circles indicate the ventral-most yR7s that express rh3^prom>GFP. For D-H, when IroC sites are mutated, expression of rh3^prom>GFP is lost in DT yR7s, similar to the loss of expression observed when Ss levels are ectopically high. As K₅₀sites are mutated, de-repression of rh3^prom>GFP expands ventrally similar to derepression of Rh3 protein in ss/tgo hypomorphic alleles. For all, rh3^prom>GFP is expressed in pR7s in all regions of the retina. DT=dorsal third, DE=dorsal equatorial, VE=ventral equatorial, VT=ventral third. Dashed yellow lines mark regions where rh3^prom>GFP is expressed in yR7s.

1. D.

   rh3^{prom IroC mut134}>GFP is expressed in pR7s only. Expression in DT yR7s is lost.

2. E.

   rh3^{prom wild type}>GFP is expressed in yR7s in the DT.

3. F.

   rh3^{prom K50 mut1}>GFP is expressed in yR7s in the DT and DE.

4. G.

   rh3^{prom K50 mut2}>GFP is expressed in yR7s in the DT, DE, and VE.

5. H.

   rh3^{prom K50 mut12}>GFP is expressed in yR7s throughout the retina (including DT, DE, VE, and VT).

Figure 7. Robust Rh4 activation requires a canonical Ss binding site

For A-D, colors indicate known cis-regulatory regions. Blue = RUS4A, Red subset of RUS4A = XRE core site (Ss/Tgo binding site), Green = RCSI (inverted in D. vir, D. moj, D. gri), Yellow = TATA box.

1. Schematic of rh4 promoter that recapitulates Rh4 protein expression. Sequence shows known critical cis-regulatory elements.

2. rh4^prom>GFP with wild type XRE recapitulates Rh4 protein expression.

3. Expression is lost with a point mutation in the XRE.

4. Sequence alignment of the rh4 promoter for 12 Drosophila species highlighting the known cis-regulatory elements. The XRE core sequence is perfectly conserved in all 12 species. Sequence alignment was from the UCSC Genome Browser (http://genome.ucsc.edu/) (Fujita et al., 2011; Kent, 2002; Kent et al., 2002).

5. rh4^prom>GFP is expressed at similar levels in the DT.

6. rh4^prom>GFP is expressed at similar levels in the VT.

7. rh4^prom>GFP is expressed at similar levels in tgo⁵ and wild type yR7s. Panel 1: GFP, Rh4, and Rh3; Panel 2: GFP alone; Panel 3: Rh4 alone and Panel 4: Rh3 alone. The white circle indicates a tgo⁵ mutant yR7 that expresses rh4^prom>GFP with both Rh3 and Rh4. The solid gray circle indicates a wild type yR7 that expresses rh4^prom>GFP with Rh4 alone. The dotted gray circle indicates a pR7 that expresses Rh3 alone.

8. Quantification of rh4^prom>GFP levels in tgo⁵ and wild type yR7s. tgo⁵ mutant yR7s (magenta) express rh4^prom>GFP at similar levels to wild type yR7s (red).

Figure 8. Model of region-specific regulation of Rh3 and Rh4 in yR7s by Ss and IroC

Green indicates Ss/Tgo activity levels; magenta indicates IroC activity levels; blue indicate Rh3 expression frequency in yR7s; and Red indicates Rh4 expression frequency in yR7s. As Ss/Tgo activity decreases (C–F) or IroC activity increases (I), Rh3 expression expands in yR7s. As Ss/Tgo activity increases (A) or IroC activity decreases (G), Rh3 expression is lost in yR7s. Despite changes in Ss/Tgo or IroC activity, the frequency of Rh4 expression is robust (A-D, G-l) with only subtle changes observed in the DT where Ss levels are low in strong ss lof (E). lof = loss-of-function. DT=dorsal third, DE=dorsal equatorial, VE=ventral equatorial, VT=ventral third.