The shape of things to come: Insulin signaling and the regulation and evolution of scaling in horned beetles

The developmental regulation of scaling relationships and its evolution underpin the extraordinary diversity of organismal shapes observed within and across taxa. Nutritional conditions critically influence scaling relationships, but do so in a highly trait- and taxon-dependent manner. Yet, how organisms regulate exaggerated growth of some tissues while shielding others from environmental fluctuations, and how this regulatory machinery itself evolves, remain poorly understood. Here we review and expand upon our decade long investigation of the insulin signaling pathway’s role in the regulation and evolution of scaling in horned beetles, in particular the genus Onthophagus. Species in this genus exhibit highly diversified degrees of nutritional plasticity, which in extreme cases mediates the development of alternate male phenotypes: fully horned fighter morphs and almost entirely hornless sneaker morphs. While horns exhibit explosive and exaggerated growth, other structures, such as genitalia, are almost nutrition insensitive. In this presentation we review and integrate key results from multiple studies aimed at (i) investigating the role of different members of the insulin signaling pathway, (ii) their interactions with other pathways, and (iii) the evolution of both functions and interactions within and beyond the genus Onthophagus. Our results document the developmental significance and evolutionary lability of insulin signaling in the developmental evolution of shape and scaling.