Insulin signaling’s role in mediating tissue-specific nutritional plasticity and robustness in the horn-polyphenic beetle Onthophagus taurus

Organisms cope with nutritional variation via developmental plasticity, adjusting trait size to nutrient availability for some traits while enabling others to develop in a nutritionally robust manner. Yet, the developmental mechanisms that regulate organ-specific growth across nutritional gradients remain poorly understood. We assessed the functions of members of the insulin/insulin-like signaling pathway (IIS) in the regulation of nutrition sensitivity and robustness in males of the horn-polyphenic beetle Onthophagus taurus, as well as potential regulatory interactions between IIS and two other growth-regulating pathways: Doublesex and Hedgehog signaling. Using RNA interference (RNAi) we experimentally knocked down both insulin receptors (InR1, InR2) and Foxo, a growth inhibitor. We then performed morphometric measurements on horns, a highly nutrition-sensitive trait, and genitalia, a largely nutrition-insensitive trait. Finally, we used qRT-PCR to assess expression levels of Doublesex and the Hedgehog-signaling gene smoothened following IIS-RNAi. Our results suggest that nutrition responsiveness of both traits is regulated by different IIS components, which transduce nutritional conditions to both Doublesex and Hedgehog pathways, albeit via different IIS pathway members. Combined with previous studies our findings suggest that separate origins of trait exaggeration among insect lineages were enabled through the independent cooption of IIS, yet via reliance on different components therein.