Secreted morphogens such as the Drosophila TGF-beta homolog Decapentaplegic (Dpp) are thought to spread through target tissues and form long-range concentration gradients providing positional information. Using a GFP-Dpp fusion, we monitored a TGF-beta family member trafficking in situ throughout the target tissue and forming a long-range concentration gradient. Evidence is presented that long-range Dpp movement involves Dpp receptor and Dynamin functions. We also show that the rates of endocytic trafficking and degradation determine Dpp signaling range. We propose a model where the gradient is formed via intracellular trafficking initiated by receptor-mediated endocytosis of the ligand in receiving cells with the gradient slope controlled by endocytic sorting of Dpp toward recycling versus degradation.

During development, cells acquire positional information by reading the concentration of morphogens. In the developing fly wing, a gradient of the transforming growth factor-beta (TGF-beta)-type morphogen decapentaplegic (Dpp) is transduced into a gradient of concentration of the phosphorylated form of the R-Smad transcription factor Mad. The endosomal protein Sara (Smad anchor for receptor activation) recruits R-Smads for phosphorylation by the type I TGF-beta receptor. We found that Sara, Dpp, and its type I receptor Thickveins were targeted to a subpopulation of apical endosomes in the developing wing epithelial cells. During mitosis, the Sara endosomes and the receptors therein associated with the spindle machinery to segregate into the two daughter cells. Daughter cells thereby inherited equal amounts of signaling molecules and thus retained the Dpp signaling levels of the mother cell.

The availability of the full Drosophila genomic DNA sequence prompts the development of a method to efficiently obtain mutations in genes of interest identified by their sequence homologies or biochemically. To date, molecularly characterized mutations have been generated in around 6000 of the ∼15,000 annotated fly genes, of which around one-third are essential for viability. To obtain mutations in essential and nonessential genes of interest, we took a reverse genetics approach, based on the large-scale detection of point mutations by Cel-I-mediated heteroduplex cleavage. A library of genomic DNA from 2086 EMS-mutagenized lines was established. The library was screened for mutations in three genes. A total of 6.1 Mb were screened, and 44 hits were found in two different mutagenesis conditions. Optimal conditions yielded an average of one mutation every 156 kb. For an essential gene tested, five of 25 mutations turned out to cause lethality, confirming that EMS mutagenesis leads to high frequency of gene inactivation. We thereby established that Cel-I-mediated TILLING can be used to efficiently obtain mutations in genes of interest in Drosophila