In the Light of Evolution Volume I: Adaptation and Complex Design (2007) / Chapter Skim
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Pages 257-282
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From page 257... ...
Recent comparative studies using DNA sequence based phylogenies have begun to reconstruct the historical pat terns 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 environ mental variables, such as nutrition.
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From page 258... ...
And once present, how were these structures modified so dramatically in form? in this chapter, we approach these questions from two vantages, comparative phylogenetic studies of horn evolution and developmental studies of the regulation of horn growth, and show that these disparate biological perspectives converge on the same basic conclusions regarding horn evolution: beetle horns do not appear to be difficult structures to gain or lose, and they appear capable of rapid and radical changes in form.
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From page 259... ...
. Thus, beetle horns are conspicuous morphological structures of known functional significance, and the more than a century of interest and observation of these animals, combined with the recent behavioral studies listed above, provide a rich ecological context for the historical and developmental studies we are about to describe.
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From page 260... ...
Based on these observations we recently proposed that the ancestor to the scarabs may have had both horns, and mechanisms for suppressing horn growth (horn dimorphism)
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From page 261... ...
characteristic of more typical horned scarabs. These observations led us recently to propose that the earliest scarabs may have had horns, as well as the developmental capacity to suppress horn growth facultatively (i.e., horn dimorphism; Fig.
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From page 262... ...
, but all studies agree that beetle horns have arisen and been lost many, many times. Conclusion 2: Beetle Horns Change Rapidly and Dramatically in Form one recent attempt to characterize the vast diversity of scarab horn morphologies revealed the following four principal trajectories of horn evolution (emlen et al., 2006)
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From page 263... ...
male dimorphism and sexual dimorphism. in most species with male dimorphism, males smaller than a critical, or threshold, body size dis pense with horn production, resulting in horn lengths that scale according to a very different relationship than in large males.
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From page 264... ...
. Combined, these four trajectories account for most of the extant diversity in horn forms.
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From page 265... ...
in these respects, beetle horns develop in a way very similar to the traditional appendages in beetles and other insects (e.g., wings or legs) , and it now appears that similar mechanisms may be involved.
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From page 266... ...
2 / Douglas J Emlen et al.
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From page 267... ...
horn development can be divided into two stages: a period of horn growth when horn cell proliferation occurs, and a period of horn remodeling. The top box shows horn growth.
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From page 268... ...
Beetle horns do not have segments or joints, but they do have an axis of outgrowth. it now appears that horns form by deploying the outgrowth portion of the patterning cascade (Moczek and nagy, 2005; emlen et al., 2006; Moczek, 2006a,b)
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From page 269... ...
All evidence to date suggests that beetle horns form their axis of outgrowth using this same basic patterning pathway. eight patterning genes are already known to be expressed in horn discs during the period of disc cell proliferation, and most (but interestingly, not all)
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From page 270... ...
Beetle horn growth depends critically on larval access to nutrition (emlen, 1994; iguchi, 1998; Moczek and emlen, 1999; hunt and simmons, 2000; Karino et al., 2004)
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From page 271... ...
their growth was to this variation in nutrition. Male horns were the most sensitive, and horn lengths were >10-fold longer in the largest individuals than they were in the smallest individuals (females of this species do not produce enlarged horns, and the height of the corresponding head region is indicated by the gray squares)
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From page 272... ...
22 / Douglas J Emlen et al.
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From page 273... ...
. one of the most exciting recent discoveries regarding beetle horn development was the observation by Moczek and colleagues (Moczek, 2006b; Moczek et al., 2006a)
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From page 274... ...
, suggesting that the evolution of a branched horn shape in this lineage resulted primarily from genetic modifications to the patterning pro cesses that control cell proliferation (horn growth)
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From page 275... ...
. in the next section, we use these developmental mechanisms as a framework for beginning to predict how diversity in beetle horns may have been generated.
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From page 276... ...
Changes in the Expression of Patterning Genes: Evolution of Horn Location and Shape? if the patterning of beetle horns works in the same way that it does in other insect appendages such as Drosophila legs, then the patterning genes will have precise domains of expression that map to specific parts of the final structure.
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From page 277... ...
For these reasons, the limb-patterning pathway has been a major focus for studies of the developmental basis for morphological evolution in arthropods. in the case of beetle horns, we think that subtle changes in the levels of expression of these patterning genes may underlie at least two types of changes in horn form, the evolution of horn location and horn shape (emlen et al., 2006)
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From page 278... ...
. it is tempting to speculate that the evolution of extreme sizes in these charismatic traits resulted from something as simple as genetic changes to the sensitivity of their cells to insulin or other nutritiondependent physiological signals.
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From page 279... ...
Genetic changes to the spatial domains of expression of patterning genes could underlie evolutionary changes in horn shape through their effects on the relative locations and amounts of cell death in pupal horns, rather than (or in addition to) any effects that they may have on proliferation.
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From page 280... ...
These studies also reveal a variety of mechanisms for shutting off horn growth. CONCLUSIONS even this preliminary examination of the mechanisms of beetle horn development reveals a great deal about their capacity for evolution.
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From page 281... ...
We now suspect that subtle genetic changes in just a few elements within these mechanisms might be sufficient to generate all four of the principal trajectories of horn evolution: changes in horn location, shape, allometry and dimorphism. one hundred and thirty five years ago, Darwin noted that sexual selection appeared to have acted ‘‘especially effectively'' in scarab beetles (Darwin, 1871, p.
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