Fig. 4. spen potentiates Wg-dependent repression of the morphogenetic furrow. (A-C) SEM images of adult eyes of wild-type (A), dpp^blk (B) and spen^k07721, dpp^blk/spen^k13624, dpp^blk (C) flies. (D-L) Confocal images of eye imaginal discs with the following genotypes: spen^k07721-rev/spen^k13624-rev (D,G,J), spen^k07721-rev, dpp^blk/spen^k13624-rev, dpp^blk (E,H,K) and spen^k07721, dpp^blk/spen^k13624, dpp^blk (F,I,L). The k07721-rev and k13624-rev alleles are homozygous viable revertants of the spen P-element alleles k07721 and k13624 resulting from precise excisions. Late third instar eyes (D-I) and early third instar eyes (J-L) are stained for Wg (D-F,J-L) and Elav (G-L). Ectopic Wg in dpp^blk eyes inhibits the furrow, thereby reducing Elav-positive photoreceptors (compare G with H) and resulting in a small eye phenotype (compare A with B). Reduced spen dosage in transheterozygotes of hypomorphic alleles increases the number of photoreceptors (I) and the adult eye size (C); the higher than normal Wg level in dpp^blk eyes (compare D with E and J with K) is unaffected in the dpp^blk, spen mutants at early third instar (L) and slightly reduced at late third instar (F). (M,N) Confocal images of third instar eye imaginal discs showing Elav (red) and either lacZ (M, green) or GFP (N, green). Clones of pygo¹⁰ (M) and spen^14C2 (N) (marked by the absence of lacZ or GFP) were generated by ey-FLP. pygo clones at the edge of the eye block Wg-dependent furrow inhibition and produce ectopic photoreceptors (arrow in M). By contrast, ectopic photoreceptors do not form in spen clones at similar positions (arrow in N; spen^14C2: n=16; spen³: n=11).

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