Chapter 17: Drosophila Imaginal Discs

Central to this process are imaginal discs, which are specialized epithelial clusters specified during embryogenesis that grow extensively throughout the three larval instars. The text details how the hormonal balance between ecdysone and juvenile hormone regulates molting and the final metamorphic surge that transforms these undifferentiated discs into complex adult structures like wings, legs, and antennae. Significant attention is given to the advanced genetic toolkits used to study these processes, such as the GAL4/UAS system for ectopic gene expression and mitotic recombination techniques like the FLP/FRT system, which allow researchers to generate and track labeled cell clones to observe lineage and gene function. The chapter introduces the vital concept of developmental compartments, defining them as restricted domains governed by "selector genes"—such as engrailed for the posterior and apterous for the dorsal regions—which establish invisible boundaries that cells do not cross. Regional patterning within these discs is explained through the action of morphogen gradients, where signaling molecules like Hedgehog and Decapentaplegic (Dpp) create concentration thresholds that instruct cells on their spatial identity. Furthermore, the text explores the regenerative potential of imaginal discs, describing how injury can trigger wound healing or even transdetermination, where a disc's identity switches to a different body part due to the misexpression of master regulators like vestigial. The conclusion covers modern research into morphogen transport mechanisms, contrasting receptor-mediated endocytosis for Dpp with other pathways, and explains planar cell polarity, the system ensuring that individual cell structures, like hairs, are oriented correctly across the tissue surface.