| Pattern formation is the fundamental process by which cells organize into highly ordered spatial arrangements of diverse cell types. A central molecular component of the pattern formation mechanism in metazoans is the Notch signalling pathway. The Notch gene encodes a transmembrane receptor that functions in an evolutionarily conserved cell interaction pathway controlling fate determination. For example, Notch is required in the Drosophila wing to induce wing margin fate and to restrict wing vein fate within vein competent regions. In this thesis, I will describe a genetic and molecular characterization of hephaestus (heph), and will present evidence that heph is required to repress Notch activity in the wing imaginal disc. The heph complementation group is defined by four lethal alleles that fail to complement the previously identified heph allele, ms(3)heph2. All five heph alleles map to a single transcription unit encoding a putative RNA binding protein homologous to vertebrate polypyrimidine-tract binding protein (PTB). This is the first genetic analysis of PTB in any organism and the first evidence that it plays a role in the Notch signalling pathway. The phenotypes of heph in genetic mosaics, where patches of heph mutant tissue (clones) are surrounded by wildtype tissue, were analyzed to study the normal role of heph during development. Near the normal wing margin, heph clones in any of the four distal wing compartments induce ectopic wing margin bristles that retain their normal compartmental identities. heph clones in the wing blade autonomously disrupt the differentiation of veins, and this depends on the Notch ligand Delta. These phenotypes suggest that heph normally represses activity of the Notch signalling pathway. In support of this hypothesis, heph clones near the presumptive wing margin autonomously express the Notch target genes wingless and cut, and induce expression of the WG target gene achaete (and thus wing margin fate) in surrounding tissue. heph clones anywhere in the wing disc have higher levels of activated Notch compared to surrounding tissue, indicating a general role for heph in down-regulating Notch activity. |
