Assessing the evolutionary lability of insulin signaling in the regulation of nutritional plasticity across traits and species of horned dung beetles

Nutrition-dependent growth of sexual traits is a major contributor to
phenotypic diversity and a large body of research documents insulin
signaling as a major regulator of nutritional plasticity. However,
findings across studies raise the possibility that the role of individual
components within the insulin signaling pathway diverge in function among
traits and taxa. Here, we use RNAi-mediated transcript depletion in the
gazelle dung beetle to investigate the functions of forkhead box
O (Foxo) and two paralogs of the insulin receptor (InR1 and InR2) in
shaping nutritional plasticity in polyphenic male head horns, exaggerated
fore legs, and weakly nutrition-responsive genitalia. Our functional
genetic manipulations led to three main findings: FoxoRNAi reduced the
length of exaggerated head horns in large males, while neither InR1 nor
InR2 knock-downs resulted in measurable horn phenotypes. These results are
similar to those documented previously for another horned dung beetle
species (Onthophagus taurus), but in stark contrast to findings in
rhinoceros beetles. Secondly, knockdown of Foxo, InR1, and InR2 led to an
increase in the intercept or slope of the scaling relationship of
genitalia size. These results are in contrast even to results documented
previously for O. taurus. Lastly, while FoxoRNAi reduces male
forelegs in D. gazella and O. taurus, the effects of InR1 and InR2
knockdowns diverged across dung beetle species. Taken together, our
results add to the growing body of literature indicating that despite
insulin signaling’s conserved role as a regulator of nutritional
plasticity, the functions of its components may diversify among traits and
species.