Figure 4
Two pathways putatively involved with the development and evolution of beetle horns (and other appendages/outgrowths of arthropods). The limb-patterning pathway (blue) acts largely autonomously within the cluster of epidermal cells that will form the horn (ie within the horn ‘imaginal disc’). Patterning results from a hierarchical cascade of local signaling interactions that delineate precise regions within the structure, and coordinate cell proliferation within those regions (see text for references). Elements shown are all involved with the formation of an axis of outgrowth (proximal → distal), and altered expression of any of these pathway elements are predicted to change the dimensions/proportions of the resulting structure (eg the length and shape of the horn), as well as the precise location of the structure (which cells become the distal tip; dark blue ring at lower right). Cell proliferation in insect imaginal discs is modulated in response to larval nutrition. The Insulin pathway (left) incorporates long-range (whole-animal) circulating signals that, when bound to receptors within the different imaginal discs, control the rate of proliferation that occurs in the disc. Altered levels of expression of elements in this pathway changes how specific traits respond to the nutrition environment, and are predicted to change trait allometry. Reduced signaling in either of these pathways would dramatically alter horn growth, and both are candidate mechanisms for dimorphism (male dimorphism, sexual dimorphism) in horn expression. Abbreviations: limb patterning pathway, ubx, ultrabithorax; en, engrailed; hh, hedgehog; wg, wingless; dpp, decapentaplegic; dac, dachshund; dll, distal-less; egfr, epidermal growth factor receptor; al, aristaless; Insulin pathway, PTEN, protein tyrosine phosphatase; PI3K, phosphatidylinositol-3-OH-kinase; PIP3, phosphatidylinositol (3,4,5) triphosphate; PKB (=Akt), protein kinase B; FOXO, forkhead-related transcription factor; 4E-BP, 4E-binding protein; S6, S6 kinase.
