Regulation of Anisotropic Tissue Growth by Two Orthogonal Signaling Centers.

The Drosophila wing has served as a paradigm to mechanistically characterize the role of morphogens in patterning and growth. Wingless (Wg) and Decapentaplegic (Dpp) are expressed in two orthogonal signaling centers, and their gradients organize patterning by regulating the expression of well-defined target genes. By contrast, graded activity of these morphogens is not an absolute requirement for wing growth. Despite their permissive role in regulating growth, here we show that Wg and Dpp are utilized in a non-interchangeable manner by the two existing orthogonal signaling centers to promote preferential growth along the two different axes of the developing wing. Our data indicate that these morphogens promote anisotropic growth by making use of distinct and non-interchangeable molecular mechanisms. Whereas Dpp drives growth along the anterior-posterior axis by maintaining Brinker levels below a growth-repressing threshold, Wg exerts its action along the proximal-distal axis through a double repression mechanism involving T cell factor (TCF).