Mechanisms of exaggerated growth in beetle weapons

Our research focuses on the genetic mechanisms of conditional-expression and trait exaggeration in weapons of sexual selection. We study these mechanisms of weapon growth in three different lineages of scarab beetles, encompassing the three principle origins of exaggerated weapons of sexual selection in this superfamily: horns of dung beetles (Scarabaeinae) and rhinoceros beetles (Dynastinae), and mandibles of stag beetles (Lucanidae). In the process, we have developed genomic tools and resources for all three lineages. By examining how traits grow to extreme proportions, we have begun to elucidate the underlying physiological and genetic pathways regulating the rate of trait growth, and coupling growth with environmental factors such as nutrition and with individual body condition.  We have focused on insulin and insulin-like growth factors because they have been shown to be essential for regulating tissue growth and body size and because their role Has been largely conserved among metazoans. We predicted that increased cellular sensitivity to the insulin/IGF pathway within specific trait primordia might be a mechanism for the evolution of disproportionate, or exaggerated, growth in the showy ornaments and weapons of sexual selection. Here, we present results from our studies of the insulin receptor in the rhinoceros beetle and stag beetle which support the hypothesis that the insulin/IGF pathway contributes to exaggerated growth of animal weapons.