Wingless effects mesoderm patterning and ectoderm segmentation events via induction of its downstream target sloppy paired

Inactivation of either the secreted protein Wingless (Wg) or the forkhead d omain transcription factor Sloppy Paired (Slp) has been shown to produce si milar effects in the developing Drosophila embryo. In the ectoderm, both ge ne products are required for the formation of the segmental portions marked by naked cuticle. In the mesoderm, Wg and Sip activities are crucial for t he suppression of bagpipe (bap), and hence visceral mesoderm formation, and the promotion of somatic muscle and heart formation within the anterior po rtion of each parasegment, In this report, we show that, during these devel opmental processes, wg and sip act in a common pathway in which sip serves as a direct target of Wg signals that mediates Wg effects in both germ laye rs. We present evidence that the induction of sip by Wg involves binding of the Wg effector Pangolin (Drosophila Lef-1/TCF) to multiple binding sites within a Wg-responsive enhancer that is located in 5' flanking regions of t he slp1 gene. Based upon our genetic and molecular analysis, we conclude th at Wg signaling induces striped expression of Sip in the mesoderm, Mesoderm al Sip is then sufficient to abrogate the induction of bagpipe by Dpp/Tinma n, which explains the periodic arrangement of trunk visceral mesoderm primo rdia in wild type embryos. Conversely, mesodermal Sip is positively require d, although not sufficient, for the specification of somatic muscle and hea rt progenitors. We propose that Wg-induced slp provides striped mesodermal domains with the competence to respond to subsequent sip-independent Wg sig nals that induce somatic muscle and heart progenitors. We also propose that in wg-expressing ectodermal cells, sip is an integral component in an auto crine feedback loop of Wg signaling.