title: Endocrine Control of Exaggerated Trait Growth in Rhinoceros Beetles.
abstract: Juvenile hormone (JH) is a key insect growth regulator frequently involved in modulating phenotypically plastic traits such as caste determination in eusocial species, wing polymorphisms in aphids, and mandible size in stag beetles. The jaw morphology of stag beetles is sexually-dimorphic and condition-dependent; males have larger jaws than females and those 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 development, and ectopic application of fenoxycarb (JH analog) to small males can induce mandibular growth similar to that of larger males. What remains unknown is whether JH regulates condition-dependent trait growth in other insects with extreme sexually selected structures. 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 nutritional state during larval development. Like stag beetles, male rhinoceros beetles receiving copious food produce disproportionately large horns for their body size compared with males under restricted diets. We show that JH titers are correlated with body size during the late feeding and early prepupal periods, but this correlation disappears by the late prepupal period, the period of maximum horn growth. While ectopic application of fenoxycarb during the third larval instar significantly delayed pupation, it had no effect on adult horn size relative to body size. Fenoxycarb application to late prepupae also had at most a marginal effect on relative horn size. We discuss our results in context of other endocrine signals of condition-dependent trait exaggeration and suggest that different beetle lineages may have co-opted different physiological signaling mechanisms to achieve heightened nutrient-sensitive weapon growth.
authors.affiliation: *Department of Entomology, Washington State University, Pullman, WA 99164 USA.
authors.forename: R
authors.initials: R
authors.lastname: Zinna
authors.affiliation: **Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Aichi 464-8601, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Gotoh
authors.affiliation: U.S. Department of Agriculture, Arid-Land Agricultural Research Center, Maricopa, AZ 85138 USA.
authors.forename: C S
authors.initials: CS
authors.lastname: Brent
authors.affiliation: Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA, 50011, USA.
authors.forename: A
authors.initials: A
authors.lastname: Dolezal
authors.affiliation: Department of Biology, Gonzaga University, Spokane, WA 99258 USA.
authors.forename: A
authors.initials: A
authors.lastname: Kraus
authors.affiliation: Division of Evolutionary Developmental Biology, National Institute for Basic Biology, Okazaki, Aichi 444-8585, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Niimi
authors.affiliation: Division of Biological Sciences, University of Montana-Missoula, Missoula, MT 59812, USA.
authors.forename: D
authors.initials: D
authors.lastname: Emlen
authors.forename: L C
authors.initials: LC
authors.lastname: Lavine
mesh_terms: D000818:Animals; D001517:Coleoptera / Q000033:anatomy & histology* / Q000187:drug effects* / Q000254:growth & development; D005260:Female; D006458:Hemolymph / Q000737:chemistry; D007605:Juvenile Hormones / Q000097:blood / Q000494:pharmacology*; D007814:Larva / Q000187:drug effects / Q000254:growth & development; D008297:Male; D010641:Phenotype; D048448:Phenylcarbamates / Q000097:blood / Q000494:pharmacology*; D011679:Pupa / Q000187:drug effects / Q000254:growth & development; D012727:Sex Characteristics
chemical_list: D007605:Juvenile Hormones; D048448:Phenylcarbamates; C052034:fenoxycarb