The progeny of wingless-expressing cells deliver the signal at a distance in Drosophila embryos

Pattern formation In developing animals requires that cells exchange signal s mediated by secreted proteins. How these signals spread is still unclear. It is generally assumed that they reach their target site either by diffus ion or active transport (reviewed in [1,2]). Here, we report an alternative mode of transport for Wingless (Wg), a member of the Wnt family of signali ng molecules. In embryos of the fruit fly Drosophila, the wingless (wg) gen e is transcribed in narrow stripes of cells abutting the source of Hedgehog protein. We found that these cells or their progeny are free to roam towar ds the anterior. As they do so, they no longer receive the Hedgehog signal and stop transcribing wg. The cells leaving the expression domain retain in herited Wg protein In secretory vesicles, however, and carry it forwards ov er a distance of up to four cell diameters. Experiments using a membrane-te thered form of Wg showed that this mechanism is sufficient to account for t he normal range of Wg. Nevertheless, evidence exists that Wg can also reach distant target cells independently of protein inheritance, possibly by res tricted diffusion. We suggest that both transport mechanisms operate In wil d-type embryos.