Wnt signalling molecules are thought to direct the development of an organism by spreading through tissues. But flies grow with almost normal appendages even when their main Wnt protein cannot move.
The Drosophila (fruitfly) protein Wingless (Wg) is the prototype member of the Wnt family of proteins, which regulate tissue patterning and growth during development. Wg is thought to act as a morphogen — a protein that forms concentration gradients as it spreads from its site of synthesis and that regulates gene expression as a function of its concentration.
Wing formation in flies expressing a form of Wg that is tethered to the cell membrane, in place of the secreted protein. Normal wing morphology, although development is delayed and the final wings are smaller than those of normal flies. Morphogen regulation of target genes depends on the physical distance from the morphogen-secreting cell population, such that the levels of this molecule provide a genetic reading of position, a key issue in morphogenesis. The best examples of morphogens come from Drosophila: the secreted molecules Hedgehog, Decapentaplegic (Dpp) and Wingless (Wg) have been identified as morphogens, and for Dpp and Wg there is compelling evidence that they act at long range. It follows from the very definition of a morphogen that the spread of the molecule is an essential component of its function. (Nature 505, 162–163 (09 January 2014) doi:10.1038/nature12848)