The role of juvenile hormone in the growth of the condition-dependent weapons of rhinoceros beetles (Trypoxylus dichotomus)

Juvenile hormone (JH) has diverse functions in insects and is critical in the control of plastic phenotypes. This hormone is known to mediate the condition dependent expression of mandibles in male stag beetles. For example, large male stag beetles have higher levels of JH than small males during the prepupal period.  Ectopic application of fenoxycarb (a JH analogue) to small males at the prepupal stage caused disproportionate mandible growth. In this study, we tested the hypothesis that JH mediates the condition-dependent expression of the elaborate horns of the Asian rhinoceros beetle, Trypoxylus dichotomus. The sexually dimorphic horns of this beetle are sensitive to the nutrition condition of the developing larva. Males receiving large amounts of food produce disproportionately large horns for their body size compared to males that receive restricted amounts of food. Unlike the stag beetle, we found it is the larval JH titer that correlates with body size in the rhinoceros beetle, instead of the prepupal JH titer. Ectopic application of fenoxycarb during the third larval instar significantly delayed pupation time, but had no effect on adult horn size relative to body size. The role of JH in the rhinoceros beetle is being further investigated through treatments with fenoxycarb during the prepupal period. Knockdown of key JH-pathway genes using RNAi technology will also facilitate our understanding of the proximate mechanism behind nutritionally plastic responses. Our research demonstrates that different beetle lineages have evolved different mechanisms to respond to changes in nutrition.