Fig. 3. Notch signaling specifies cell fate and behavior. (A) A schematic of inductive Notch signaling, which typically occurs between non-equivalent cell populations. In this case, the blue cells signal to adjacent white cells to induce a new cell fate or change their behavior (black cells). (B) The failure of inductive Notch signaling results in the absence of this cell fate or behavior, whereas excessive Notch signaling has the reciprocal effect. (C-E) Notch signaling promotes Drosophila wing growth. (C) Wild-type adult wing. (D) Wing containing large Su(H) mutant clones in which loss of the CSL transcription factor causes notching (stars), owing to the failure to specify the wing margin during earlier development. (E) Wing containing clones of cells that misexpress Delta and that induce a large wing overgrowth (image courtesy of Jose F. de Celis). (F,G) Notch signaling is involved in the segmentation clock. Shown are stage 3S zebrafish embryos stained for the Notch ligand DeltaC (images courtesy of Clarissa Henry). (F) In a wild-type embryo, DeltaC oscillates in stripes (arrows) that correlate with the partitioning of somites. (G) An embryo in which the bHLH repressor-encoding Notch target genes her1 and her7 have been inhibited by injection of morpholinos. The oscillatory pattern of DeltaC expression is lost (bracket), and such embryos develop abnormal somites. (G,H) Notch signaling promotes germline proliferation. Shown are C. elegans gonads stained with DAPI (blue) to reveal nuclei (images courtesy of Tim Schedl). (H) In the wild-type gonad, mitotic nuclei are localized to the distal region, while the remainder of the gonad is meiotic and produces germ cells (oocytes). (I) A gain-of-function mutant in the GLP-1 receptor (Notch) shows a `tumorous' phenotype in which mitotic cells are found throughout the gonad and germ cells fail to differentiate.