On the origin and evolutionary diversification of beetle horns

*Division of Biological Sciences, University of Montana, Missoula, MT 59812; and
^‡Department of Entomology, Washington State University, Pullman, WA 99164

Abstract

Many scarab beetles produce rigid projections from the body called horns. The exaggerated sizes of these structures and the staggering diversity of their forms have impressed biologists for centuries. Recent comparative studies using DNA sequence-based phylogenies have begun to reconstruct the historical patterns of beetle horn evolution. At the same time, developmental genetic experiments have begun to elucidate how beetle horns grow and how horn growth is modulated in response to environmental variables, such as nutrition. We bring together these two perspectives to show that they converge on very similar conclusions regarding beetle evolution. Horns do not appear to be difficult structures to gain or lose, and they can diverge both dramatically and rapidly in form. Although much of this work is still preliminary, we use available information to propose a conceptual developmental model for the major trajectories of beetle horn evolution. We illustrate putative mechanisms underlying the evolutionary origin of horns and the evolution of horn location, shape, allometry, and dimorphism.

Footnotes

^†To whom correspondence should be addressed. E-mail: doug.emlen{at}mso.umt.edu
Author contributions: D.J.E. and L.C.L. designed research; D.J.E., L.C.L., and B.E.-C. performed research; and D.J.E. wrote the paper.
This paper results from the Arthur M. Sackler Colloquium of the National Academy of Sciences, “In the Light of Evolution I: Adaptation and Complex Design,” held December 1–2, 2006, at the Arnold and Mabel Beckman Center of the National Academies of Sciences and Engineering in Irvine, CA. The complete program is available on the NAS web site at www.nasonline.org/adaptation_and_complex_design.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/cgi/content/full/0701209104/DC1.
Abbreviation:

InR,

insulin receptor.