D.J.E. unpublished data)]. Ongoing research involves experiments that test for functional roles for these genes (e.g., by knocking down transcript abundance, using RNA interference methods), and more comprehensive examinations of the patterns of expression of these genes in individuals that differ in horn size and in species that differ in horn form. Future studies are likely to resolve this process in greater detail, but it is probably safe to conclude that the first stage of building a beetle horn involves the deployment of the outgrowth module of the appendage patterning process.
The patterning of insect imaginal discs is not the whole story. Anyone who has reared insects in captivity knows that trait sizes are almost always phenotypically plastic. In particular, they are sensitive to nutrition. Somehow, the basic patterning and growth of structures must be modified in response to the conditions animals encounter as they develop, including and especially the larval nutritional environment.
Beetle horn growth depends critically on larval access to nutrition (Emlen, 1994; Iguchi, 1998; Moczek and Emlen, 1999; Hunt and Simmons, 2000; Karino et al., 2004). Both horn size and body size are sensitive to variation in nutrition, with the consequence that there is a coupling of the amount of horn growth with overall body size. Iterated across a number of different individuals developing under a range of nutritive conditions and environments, the result of this phenotypic plasticity is allometry, the scaling of body parts with body size (Fig. 14.7). This highlights an important, but slightly counterintuitive point: nutrition-dependent phenotypic plasticity and allometry are related. In insects at least, they both result from physiological mechanisms that modulate the amount of trait growth in response to nutrition (Stern and Emlen, 1999; Emlen and Nijhout, 2000; Emlen and Allen, 2004).
We illustrate this relationship with data from a diet-manipulation experiment, in which individuals from a number of different maternal lines were divided among either a high (large food amount) or a low (reduced food amount) nutrition environment. Larvae given poor nutrition emerged into adults with small body sizes that had short horns and legs and tiny wings (Fig. 14.7, closed circles). From the same families, siblings given large food amounts matured into adults with much larger body sizes that had much longer horns, legs, and wings (open circles in Fig. 14.7). Each of these traits is phenotypically plastic, because trait size is sensitive to the larval nutritional environment. In all cases, the magnitude of the plastic developmental response is reflected in the resulting population-level trait size versus body size allometries. Traits that