Fig. 1.

Ewg is required for photoreceptor apoptosis in response to Apc1 loss. (A-F) Cross-sections through adult retinas. (A) Eight photoreceptors are present in each ommatidium with seven photoreceptors visible in this section, arrayed in a trapezoidal pattern. (B) Upon Apc1 loss, all photoreceptors undergo apoptosis. The pigment cell lattice remains intact. (C-E) Photoreceptor apoptosis is prevented in hemizygous ewg^P1 (C), transheterozygous ewg¹/ewg² (D) and ewg^P1/ewg² mutants (E). (F) Exogenous ewg expression driven by the elav promoter restores apoptosis in ewg^P1; Apc1^Q8 double mutants. (G) Schematic representation of the Ewg protein. A C-to-T substitution is found in ewg^P1, resulting in an arginine to cysteine substitution in amino acid 171. In ewg², an early stop codon at amino acid 178 is caused by substitution of C to T; no mutation was detected in the ewg-coding region in the ewg¹ allele (G. Gill, personal communication).