the rate of growth of that trait overall and enhance the sensitivity of that trait to nutrition. Thus, it should lead to increased nutrition-dependent phenotypic plasticity in horn growth, a steeper population-level horn allometry slope, and, in the best-fed individuals, a disproportionately larger final horn size.
It is noteworthy that all of these properties (enhanced nutrition-sensitivity, steep allometry slopes, and disproportionately large final trait sizes) are characteristic of the most extreme and exaggerated morphological structures in insects (Emlen and Nijhout, 2000) and of the enlarged ornaments and weapons of sexual selection in general (Andersson, 1994; Cotton et al., 2004). It is tempting to speculate that the evolution of extreme sizes in these charismatic traits resulted from something as simple as genetic changes to the sensitivity of their cells to insulin or other nutrition-dependent physiological signals.
One way to begin to test these ideas involves a comparison of insulin sensitivity across the different traits within a species. We predict that traits that have steep and positive allometry slopes should be exquisitely sensitive to insulin signals (e.g., wings, horns). Other traits that are insensitive to nutrition and have shallow allometry slopes should be relatively insensitive to insulin signals (e.g., the genitalia). What we are suggesting is that the degree of phenotypic plasticity or canalization of trait expression could result from disc-specific differences in their sensitivity to circulating insulin signals (Shingleton et al., 2007).
In Drosophila trait differences in nutrition-dependent plasticity and allometry result at least in part from disc-specific differences in their responsiveness to insulin signals. Recent experiments by Shingleton et al. (2005) showed that traits like wings were sensitive to both insulin and to perturbations to the InR, whereas the genitalia were not. Growth of the genitalia was almost entirely unaffected by perturbations to the InR. This important study confirmed that activity of the InR pathway does affect trait allometry. Results from several other studies where genetic perturbations to elements of this pathway were examined show this as well (e.g., Goberdhan and Wilson, 2002; Mirth et al., 2005).
We have used quantitation of relative transcript abundances of the InR gene as our first measure of insulin pathway activity in beetle imaginal discs, and our preliminary results indicate that horn, leg, wing, and genital discs differ predictably in their relative activities of this pathway during the period of disk growth (L.C.L. and D.J.E., unpublished results). Reduced activity of this pathway also appears to be one of the mechanisms used by scarabs to truncate horn growth, in this case, in the horn discs of small males and females of the species Onthophagus nigriventris (Emlen et al., 2006; and L.C.L. and D.J.E., unpublished data). Consequently, the insulin pathway now appears a likely candidate mechanism for the development