Localized activation of this portion of the patterning pathway stimulates and coordinates the formation of a new body outgrowth. These molecular interactions define the precise location of a structure (which cells will form the distal tip of the structure) and the signals released from these focal cells direct the subsequent behavior of neighboring cells.
Like the patterning process as a whole, the outgrowth portion of the pathway is a cascade of molecular interactions that once started, unfolds to completion relatively autonomously. This means that exposing cells to the appropriate combination of signals can activate the entire module of the patterning cascade, and result in the formation of a complete (and new) body outgrowth. For example, juxtaposition of wg and dpp signals in an inappropriate region of a developing Drosophila wing disc initiates formation of a second axis of outgrowth: a new distal tip that subsequently generates a new wing (Campbell et al., 1993; Zecca et al., 1995). This results in the formation of a bifurcated double wing blade, one wing blade that is the default outgrowth, and a second wing blade that is an aberrant outgrowth generated by activating this pathway in a second region of the disc. Similar juxtaposition of these same two signals in a Drosophila leg disc can generate a second fully formed leg attached to the original leg, again resulting in a bifurcated final structure (Diaz-Benjumea et al., 1994; Gibson and Schubiger, 1999). Although these outgrowths are generated artificially in the laboratory, they beautifully illustrate the autonomous property of this pathway, and the potential for this pathway to underlie the evolution of novel morphological structures.
Although the molecular details of this process have been especially well studied in Drosophila leg discs, the basic elements of this outgrowth portion of the patterning pathway appear to be highly conserved across different imaginal discs within a species and across taxa; indeed, all arthropod body outgrowths that have been studied to date appear to use some form of this process in their development (e.g., Panganiban et al., 1994; Jockusch et al., 2004). Thus, the outgrowth portion of the patterning pathway is an evolutionarily conserved developmental module that leads to the formation of body outgrowths in diverse taxa, including horns in beetles.
All evidence to date suggests that beetle horns form their axis of outgrowth using this same basic patterning pathway. Eight patterning genes are already known to be expressed in horn discs during the period of disc cell proliferation, and most (but interestingly, not all) of these have domains of expression consistent with their putative role in the formation of the axis of outgrowth [dung beetle (Onthophagus) horns: wingless, decapentaplegic, distal-less, daschshund, aristaless, epidermal growth factor receptor, homothorax, extradenticle (Moczek and Nagy, 2005; Moczek, 2006a; Moczek et al., 2006b; and L.C.L. and D.J.E., unpublished data); rhinoceros beetle (Dynastinae) horns: wingless, decapentaplegic (L.C.L. and