On the origin and evolutionary diversification of beetle horns

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.allometry ͉ development ͉ phenotypic plasticity ͉ sexual selection ͉ weapons

Larval growth in polyphenic salamanders: making the best of a bad lot

“…For example, in polyphenic tadpoles, carnivore versus omnivore morphs can be produced in response to several environmental variables (Pfennig 1992;Frankino and Pfennig 2001;Pfennig et al 2007), and different populations and species exposed to different environments reveal substantial variation in the production of the two morphs (Pfennig et al 2007), suggesting multiple mechanisms for morph production and maintenance. In contrast, environments with low spatiotemporal variation or in polymorphisms that are less affected by such variation, one selection mechanism may attain primacy, as is suggested by numerous studies showing 'primary' versus 'secondary' morphs, such as in alternative male morphologies (e.g., Gross 1996;Emlen et al 2005Emlen et al , 2007. In this sense, polyphenisms can be separated into those that are more extrinsically-driven by spatiotemporal variation (e.g., dispersal and trophic polyphenisms; see also Roff 1994) and those that are more intrinsically-driven by strong genotypic effects (e.g., alternative male morphologies).…”

“…Salamanders have physiological mechanisms that allow them to assess and maintain homeostasis (Feder and Burggren 1992;Kikuyama et al 2005;Stiffler 2005). Such mechanisms provide an intrinsic monitor for body condition (e.g., gut fullness and blood lipid titers in the short term, and fat storage in the long term), and indirectly allow an assessment of environmental quality (Denoël et al 2002), similar to that described in insects (e.g., Emlen et al 2007).…”

“…Polyphenisms provide model systems for the study of phenotypic variation because: (1) the trait of interest has a direct environmental component; (2) alternative phenotypes are often distinct and easily distinguished; and (3) they are likely to be a direct result of selection (Caswell 1983;Smith-Gill 1983). Documenting the ecological and evolutionary contexts that produce and maintain polyphenisms is critical for understanding their role in speciation, life history evolution, the maintenance of biodiversity, and the evolution of plasticity (West-Eberhard 1986, 2003Hazel et al 1990;Via 2001;Denoël et al 2005;Emlen et al 2007;Pfennig et al 2007Tomkins and Hazel 2007;Pfennig and McGee 2010).…”

On the origins of novelty in development and evolution

“…Beetle horns are major cuticular projections of the head and thorax and in regions in which insects normally do not produce outgrowths. (63,64) Used as weapons in male combat over females, horns are similar, and often larger, in size than more traditional appendages such as antennae, mouthparts or legs, but lack muscles, nerves or joints. However, they are not simply modified antennae, mouthparts or legs, which instead still exist alongside horns in the same organisms, and beetle horns can thus not be homologized to other structures in a straightforward manner.…”

Individual variation behind the evolution of cooperation

“…Variation also goes beyond the level of species: individuals of the same species, even if they are of the same sex, age and size, can be different [13]. Morphological polymorphisms within species, such as wing dimorphism and weaponry variability in insects, or plumage polymorphisms in birds such as the ruff (Philomachus pugnax), have been recognized for a long time [14][15][16]. Recent studies have also found consistent behavioural differences between individuals of the same species, sex, age and size, a phenomenon that is similar to human personality [17 -19].…”