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 Apc1^Q8 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 pan^ER1 allele, both apoptosis and the shortening of PR length are partially suppressed in the Apc1^Q8 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).