and evolution of trait plasticity and trait allometry in insects generally and in beetle horns specifically. Although we initially illustrated this point by examining how the different traits within a species have diverged (e.g., horns versus wings versus genitalia), it is important to recognize that this same process can also account for population and species differences in the expression of a single trait (Fig. 14.4). This process could explain evolutionary shifts in both the relative size of a trait and the evolution of extreme or exaggerated trait sizes.
Programmed cell death is an integral part of animal development and can lead to significant remodeling of appendages. Cell death is responsible for generating interdigital spaces in tetrapod limb buds (Rodriguez-Leon et al., 1999; Zuzarte-Luis and Hurle, 2002) and for creating cavities in the developing inner ear (Fekete et al., 1997). In insects, programmed cell death sculpts head morphology in flies (Lohmann et al., 2002) and remodels the outer margins of butterfly wings (Kodama et al., 1995). Interestingly, programmed cell death has also been shown to underlie sexual dimorphism and caste differences in insect wing morphology (Lobia et al., 2003; Sameshima et al., 2004; Nardi et al., 2005), a situation analogous in many respects to what Moczek et al. (Moczek, 2006b; Moczek et al., 2006b) have observed with beetle horns.
The preliminary findings of Moczek and colleagues (Moczek, 2006b; Moczek et al., 2006a) suggest that variation in the spatial domains of expression of the patterning gene dll during the prepupal (horn growth) period map to subsequent variation observed in the amount of resorption of horn tissue. The involvement of patterning signals in this process of pupal horn remodeling would be exciting, because it would suggest parallels with the molecular mechanisms involved with tissue remodeling in other taxa (e.g., Rodriguez-Leon et al., 1999; Rusconi et al., 2000; Adachi-Yamada and O’Connor, 2002), and because it would illustrate yet another route to beetle horn evolution. Genetic changes to the spatial domains of expression of patterning genes could underlie evolutionary changes in horn shape through their effects on the relative locations and amounts of cell death in pupal horns, rather than (or in addition to) any effects that they may have on proliferation. Pupal remodeling appears to be widespread, at least within the genus Onthophagus (Moczek, 2006b), and this process could lead to the carving of spaces between horns (analogous to the spaces between vertebrate limb digits) and to fine-scale sculpting of horn barbs, branches, or curves.