Fig. 5. Rescue of retinal neuronal apoptosis in the Apc1^Q8 mutant by overexpression of Apc2. Genotypes shown are as follows: (A) P[Apc2]/+; +/+; (B) Apc1^Q8/Apc1^Q8; (C) P[Apc2]/Y; Apc1^Q8/Apc1^Q8. (A) Tangential section through the eye of a fly with one extra copy of an Apc2 transgene. As in wild-type eyes, in each ommatidium there are eight photoreceptor neurons located in a highly ordered pattern. Seven neurons are seen in the plane of focus. Each group of photoreceptor cells is surrounded by a lattice of pigment cells, identified by the small, darkly stained pigment granules they contain. (B) In the homozygous Apc1^Q8 mutant, there is apoptotic death of all retinal neurons in all ommatidia. The pigment cell lattice remains intact. (C) One extra copy of the Apc2 gene is sufficient to partially prevent the neuronal cell death that is induced by Apc1 loss. The degree to which the apoptosis is prevented is greater in Apc1 mutant males than in females. As the Apc2 transgene is inserted on the X chromosome, we infer that this difference is the result of dose compensation (Kelley and Kuroda, 1995).