Endocrine control of exaggerated traits in rhinoceros beetles

Authors: ZINNA, ROBERT - Washington State University; GOTOH, HIROKI - Nagoya University; Brent, Colin; DOLEZAL, ADAM - Iowa State University; KRAUS, AURORA - Gonzaga University; NIIMI, TERUYUKI - National Institute For Basic Biology; EMLEN, DOUGLAS - University Of Montana; LAVINE, LAURA CORLEY - Washington State University

Submitted to: Integrative and Comparative Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/25/2016
Publication Date: 6/1/2016
Citation: Zinna, R., Gotoh, H., Brent, C.S., Dolezal, A., Kraus, A., Niimi, T., Emlen, D., Lavine, L. 2016. Endocrine control of exaggerated traits in rhinoceros beetles. Integrative & Comparative Biology. 56(2):247-259.

Interpretive Summary: Juvenile hormone (JH) is a key insect growth regulator insect development, and in some species controls the development of exaggerated traits like horns and mandibles. In the Asian rhinoceros beetle, males develop elaborate horns so long as they are well fed. As with other species having exaggerated traits, the horn size in this beetle is sensitive to the nutritional conditions of the developing larva. Males receiving copious food produce disproportionately large horns for their body size compared to males under restricted diets. An ARS scientist at Maricopa, AZ and collaborators show that the amount of JH circulating in the blood during early development stages is directly related to adult body size, but during the stage when horn development is occurring there is no relationship between hormone levels and beetle size. Applying a hormone mimic also fails to affect horn size, but does delay the timing of maturation. These results suggest that the Asian rhinoceros beetle may have evolved a mechanism to control horn growth that is different from those of other species with exaggerated features, and that another signaling pathway must be used to convey the information about a beetle's nutritional status that is needed to regulate development.

Technical Abstract: Juvenile hormone (JH) is a key insect growth regulator involved in modulating phenotypically plastic traits in insects such as caste determination in eusocial species, wing polymorphisms in aphids, and mandible size in stag beetle. Male stag beetles have sexually-dimorphic, condition-dependent expression of their jaws; males developing under optimum conditions are larger in overall body size and have disproportionately larger jaws than males raised under poor conditions. We have previously shown that large males have higher JH titers than small males during a specific stage of development and ectopic application of fenoxycarb (JH analogue) to small males at this stage induces mandibular growth similar to that of males with a larger body size. What remains unknown is whether JH serves as a universal regulator of exaggerated, condition-dependent trait growth in insects. 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 head horn of this beetle is sensitive to the nutritional condition of the developing larva. Males receiving copious food produce disproportionately large horns for their body size compared to males under restricted diets. We show that, like the male stag beetle, JH titers are correlated with body size during the larval and early prepupal period, but not during the late prepupal period. While ectopic application of fenoxycarb during the third larval instar significantly delayed pupation time, it had no effect on adult horn size relative to body size. In addition, fenoxycarb application during the period of maximum horn growth had no effect on pupal horn size relative to body size. We discuss our results in context of other endocrine signals of condition-dependent trait exaggeration and show how our research demonstrates that different beetle lineages have evolved different mechanisms to respond to nutrient availability.