Figure 1.

Ectopic scute expression driven by ato regulatory regions promotes photoreceptor formation. (A–D) Scanning electron microscopy of compound eyes, showing that expression of scute either behind or in front of the MF leading furrow, combined with complementary ato expression, restores some ommatidia in the ato¹ mutant (C and D). (E–H) Third instar larval eye discs stained with an antibody against Boss, showing much reduced Boss expression if scute is expressed behind (G) rather than in front of the MF leading edge (H). (I–L) Tangential sections of compound eyes, revealing ommatidia with reduced number of photoreceptors and no R8 in the ato¹ mutant expressing ato in front of and scute behind the MF leading edge (K). (A, E, and I) Wild type; (B, F, and J) ato¹ mutant expressing ato both in front of and behind the MF leading edge (3′F:5.8-ato; 5′eye-ato,ato¹); (C, G, and K) ato¹ mutant expressing ato in front of and scute behind the MF leading edge (3′F:5.8-ato; 5′eye-sc,ato¹); (D, H, and L) ato¹ mutant expressing scute in front of and ato behind the MF leading edge (3′F:5.8-sc; 5′eye-ato,ato¹). Arrowheads in J and H indicate the rare ommatidia with fewer than eight photoreceptors. Posterior is to the left. (Magnifications: A–D, ×110; E–H, ×225; I–L, ×900.)