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First published online 30 January 2008
doi: 10.1242/dev.013805

Development 135, 963-971 (2008)
Published by The Company of Biologists 2008
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Adenomatous polyposis coli is present near the minimal level required for accurate graded responses to the Wingless morphogen

Hassina Benchabane*, Edward G. Hughes*, Carter M. Takacs, Jason R. Baird and Yashi Ahmed{dagger}

Department of Genetics and the Norris Cotton Cancer Center, Dartmouth Medical School, Hanover, NH 03755, USA.


Figure 1
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  		Fig. 1. Apc1 loss results in apoptosis of photoreceptors. (A) Schematic of the retina. High levels of Wg signaling induce apoptosis of photoreceptors (orange), intermediate levels induce photoreceptors to adopt a DRA fate (pink), and low levels result in ommatidia that lack bristles (blue). (B-I) Pupal retinas were stained with antibodies against activated caspase (green) and Elav (blue). Elav is expressed in all photoreceptors. (B,C) In wild-type pupal retinas at 38 hours after puparium formation (APF), activated caspases are present only at the periphery. (D-I) In Apc1Q8 mutants, activated caspases are observed throughout the pupal retina. Caspase activation in photoreceptors is initially observed in the posterior retina at 35 hours APF (D,E), and, over the next four hours, extends to cover the entire retina (F-I).

 

Figure 2
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  		Fig. 2. Apc1 inactivation induces an ectopic dorsal rim area (DRA) fate. Pupal retinas were stained with antibodies against Homothorax (Hth; green), Elav (blue; marking all photoreceptors) and β-gal (magenta). A svp-lacZ insertion allows demarcation of the dorsoventral equator (indicated by a solid white line in B, D and F). In all panels, dorsal is oriented to the top. (A,B) In wild-type pupal retinas, hth is expressed in all inner photoreceptors in the DRA, as well as in some pigment cells and the head capsule surrounding the retina. Hth expression (dashed white line in B) is restricted to one or two rows of ommatidia at the dorsal edge of the retina, and extends up to, or one ommatidial cluster above, the equator (B). (C-F) In Apc1Q8 mutant retinas, hth is expressed throughout the dorsal half of the retina, extending up to the equator or to one row above the equator (C,D: 20 hours APF; E,F: 30 hours APF).

 

Figure 3
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  		Fig. 3. Ectopic DRA ommatidia in the Apc1 mutant retina. (A) Whole-mount adult retina of Rh3>lacZ flies stained with X-gal. Rh3 is expressed in 30% of all R7 cells, as well as in all R7 and R8 cells of the DRA ommatidia. (B) In Apc1Q8 flies heterozygous for grim, reaper and hid (Df(3L)H99), Rh3 is present in an increased number of PRs throughout the dorsal half of the retina. (C) Adult eye section from the dorsal-most regions of a wild-type retina. Ommatidia with a DRA fate are found above the solid white line, and are characterized by inner photoreceptors with enlarged rhabdomeres (green arrowheads). (D) A Df(3L)H99 mutant clone located away from the retinal periphery in the Apc1Q8 mutant. Inner PRs with enlarged rhabdomeres are observed in many ommatidia that are not restricted to the dorsal rim (green arrowheads).

 

Figure 4
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  		Fig. 4. Ectopic Wg signaling induces DRA fates in a fraction of ventral ommatidia. (A-D) Apc1 loss and ectopic Arm signaling induces expression of Hth (green) in some ventral ommatidia. Photoreceptors are marked with anti-Elav (blue). (A) In wild-type retinas, hth expression is observed in rare photoreceptors at the ventral rim (arrowheads). (B) In Apc1Q8 mutant retinas, 30 hours APF, many photoreceptors near the ventral pole express hth. (C) In otherwise wild-type retinas, ectopic expression of the IRO-C protein Ara, under the control of a GMR promoter, induces the formation of an ectopic ventral rim area with one to three rows of ommatidia expressing hth. (D) In Apc1Q8 flies, Hth is found throughout the entire retina when Ara is expressed under the control of a GMR promoter. (E-H) Expression of full-length Arm (E,F) and constitutively active ArmS10 (G,H) under the control of elav and long GMR promoters, respectively, leads to the expansion of Hth expression (green) within the entire dorsal half of the retina, as well as in a fraction of ventral ommatidia. Photoreceptors are marked with anti-Elav (blue).

 

Figure 5
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  		Fig. 5. Activation of Arm signaling leads to a morphological defect in photoreceptor length. (A) Horizontal section from a wild-type adult eye. Photoreceptors extend from the base to the apical surface of the retina. (B,C) Cross-section of wild-type adult retina. Most photoreceptors are present at the apical surface (B) and basal surface (C) of the retina. (D,E) Apical (D) and basal (E) cross-sections of an Apc1Q8 retina containing a large clone of cells homozygous mutant for grim, reaper and hid (Df(3L)H99). (F-H) In eyes expressing constitutively active ArmS10 under the control of a GMR-GAL4 driver, photoreceptor length is shortened (horizontal section in F, apical cross-section in G and basal cross-section in H).

 

Figure 6
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  		Fig. 6. Shortened PR length in the Apc1 mutant results from ectopic activation of Arm/dTCF (Pan) signaling. (A,B) Apoptosis of photoreceptors is partially suppressed in Apc1Q8 mutants by reducing the gene dosage of pan by one-half, using the pan13 null allele. However, the shortened PR morphology is not suppressed. (A) Apical section; (B) basal section. (C,D) Upon further reduction of pan activity, in flies homozygous for the hypomorphic panER1 allele, both apoptosis and the shortening of PR length are partially suppressed in the Apc1Q8 mutant. (C) Apical section; (D) basal section. (E,F) Cross-section of the dorsal-most part of a wild-type adult retina at an apical (E) and basal (F) level. In the outermost rows of the retina, where Wingless signaling is at an intermediate level, some photoreceptors do not extend the entire length of the retina (color-coded arrowhead indicates the same ommatidium at an apical and basal level).

 

Figure 7
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  		Fig. 7. Loss of Apc2 increases the width of the dorsal rim area. Second and third row ommatidia expressing Hth (green) are marked with yellow arrowheads. Photoreceptors are marked with anti-Elav (blue). The number of second and third row ommatidia expressing hth increases when Apc2 activity is severely reduced. (A) Wild-type, (B) Apc233/Apc233 and (C) Apc233/Df(3R)w6. (D) The number of second row ommatidia expressing Hth is reduced to normal levels in Apc233 mutants with two copies of an Apc2 transgene, P[Apc213.5]. (E) Average number of second row ommatidia expressing Hth for each genotype. Standard errors are shown. (F) Adult eye section from the dorsal-most retina of a homozygous Apc233 mutant. Ommatidia with a DRA fate are found above the solid white line and are characterized by inner photoreceptors with enlarged rhabdomeres (green arrowheads). There is an increased number of ommatidia with a DRA fate when compared to wild-type retinas (see Fig. 3C). (G) Schematic of the retina in wild-type, Apc1 mutant and Apc2 mutant flies. Loss of Apc1 induces apoptosis of all photoreceptors (orange). Loss of Apc2 causes an expansion of the DRA fate (pink).

 

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