Split ends is a potentiator of Wingless signaling. H. Lin , S. Cho , K.M. Cadigan. Dept of MCDB, Univ of Michigan.

   To identify new genes involved in Wg signaling, a modifier screen was done using a sensitized misexpression phenotype. At an intermediate temperature, Wg^ts expression in the eye reduces the number of inter-ommatidial bristles, enabling the detection of both suppressors and enhancers of this phenotype. Two alleles of a dominant suppressor were isolated, increasing the bristle number in heterozygotes. This suppressor was later found to be allelic to split ends, a gene previously implicated in the EGFR and Hox pathways. Misexpressing Wg in the eye blocks bristle formation as well as the expression of Achaete and Cut in pupal bristle precursors. Both the bristles and the expression of Ac and Cut are partially rescued inside spen clones, suggesting that Spen potentiates Wg signaling. To determine where Spen acts in Wg signaling, epistasis with known components of the pathway was done. Expressing activated Armadillo in the eye activates Wg signaling downstream of Arm, resulting in bristle inhibition. spen alleles dominantly suppress this phenotype, suggesting that Spen acts downstream or at the level of Arm, or it may act in another pathway involved in bristle formation. To distinguish these possibilities, other Wg readouts were examined. Ectopic Wg signaling in an eye-specific decapentaplegic mutant inhibits the morphogenetic furrow during development, resulting in a small eye. Hypomorphic spen alleles significantly increase the number of ommatidia in this background, consistent with Spen being a potentiator of Wg signaling rather than a factor specific for bristle formation. To determine whether Spen is essential for Wg signaling, readouts in other tissues were tested. Expression of Senseless along the larval wing margin is activated by Wg, and is abolished or reduced in some spen clones. However, another Wg target in the wing, Distalless, is unaffected in spen clones. Therefore, Spen potentiates, but is not essential for Wg signaling in the wing. Moreover, results in several Wg readouts during embryogenesis also suggest that Spen is not an essential factor in the Wg pathway.