arthropods as well (Truman and Riddiford, 2002). These clusters of cells behave as remarkably autonomous units, and even when removed (in vitro) or transplanted, these discs are able to complete most of their growth and development, the result of a complex series of molecular and genetic interactions that unfolds within the disc (“patterning”; see Serrano and O’Farrell, 1997; Held, 2002; Johnston and Gallant, 2002; Kojima, 2004; Weihe et al., 2005).
The full process of appendage patterning can be functionally subdivided into at least four hierarchical and relatively dissociable modules, each entailing the deployment of a specific and largely self-contained network of genetic interactions [specification of appendage identity (leg, antenna, wing, etc.), formation of an axis of outgrowth (proximal–distal), subdivision of the appendage into segments, and localization and growth of sensory structures, bristles, and hairs (Carroll et al., 2001; Held, 2002)]. The portion of this patterning process that is most relevant to beetle horn development is the formation of an axis of outgrowth. Beetle horns do not have segments or joints, but they do have an axis of outgrowth. It now appears that horns form by deploying the outgrowth portion of the patterning cascade (Moczek and Nagy, 2005; Emlen et al., 2006; Moczek, 2006a,b). We do not describe the details of this pathway here (for reviews, see Serrano and O’Farrell, 1997; Johnston and Gallant, 2002; Kojima, 2004; Weihe et al., 2005). Instead, we highlight a few properties of this pathway that are especially relevant for understanding how beetle horns develop.
In an insect appendage, such as a Drosophila leg, the expression of patterning genes is confined to specific domains within the imaginal disc. These expression domains overlap partially, but not completely, with the domains of expression of other genes in the network, and the result is a spatially explicit mosaic of molecular signals defined by the boundaries of expression of the patterning genes. Cells physically located at the intersection of two of these boundaries, because of their position, come into contact with high concentrations of several different signals, including proteins of the patterning genes hedgehog (hh), wingless (wg), and decapentaplegic (dpp), and this critical combination of molecular signals causes these cells to become active organizers of the rest of the disc. These focal cells will give rise to the eventual distal/outermost tip of the new appendage.
Once their fate has been established, these focal cells begin expressing a new suite of patterning genes. The proteins of many of these genes diffuse outwards into the surrounding cells of the disc, activating additional tiers of patterning gene expression. This process both stimulates and coordinates cell proliferation within the disc such that there is a burst of localized growth concentrated around the focal cells. The result is a folded tube of epidermis that will subsequently unfurl to form the appendage.