Chemical Ecology The Chemistry of Biotic Interaction (1995) / Chapter Skim
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The Chemistry of Defense: Theory and Practice
The Chemistry of Defense: Theory and Practice
Pages 1-16
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From page 1... ...
In insects, many defensive secretions are derived from the same amino acids used to construct proteins famong them, quinones in many beetles and cockroaches derive from tyrosine, formic acid in ants from serine, isobutyric acid in swallowtail caterpillars from isoleucine and valine, and alkyl sulfides in ants from methionine (3, 411. Presumably, secondary compounds are physiologically active in nonconspecific organisms precisely because of their secondary nature; it is to be expected that most organisms possess effective means for metabolizing, shunting around, or May Berenbaum is professor and head of the Department of Entomology at the University of Illinois, Urbana-Champaign.
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From page 2... ...
Levels of abundance are subject to environmental or developmental regulation and, unlike primary constituents, which may be present in virtually all cells of an organism, chemical defenses are typically compartmentalized, even in those cases in which the chemicals are acquired exogenously, as when sequestered from a food source. There often exists a system for external discharge, delivery, or activation, not only as a means of ensuring contact with a potential consumer but also as a means of avoiding autotoxicity until a confrontation arises; and of course these compounds are almost invariably, by virtue of structure, chemically reactive (e.g., able to be taken up by a living system, to interact with a receptor or molecular target, and to effect a change in the structure of the molecular target)
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From page 3... ...
Nonetheless, an examination of the distribution, pattern of allocation, chemical structure, and modes of action of secondary compounds in a broad cross section of organisms reveals so many striking convergences and similarities that the notion that variation in the distribution and abundance of chemicals that act as poisons results at least in part from selection by consumer organisms certainly seems tenable, if not inescapable. DISTRIBUTION OF DEFENSES One line of evidence, admittedly circumstantial, that consumers have influenced the evolution of chemical defenses is their taxonomic distri
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From page 4... ...
Hoopoes are also opportunistic feeders that consume debris along with insects and other invertebrates. It is somewhat surprising that chemical defenses are not more frequently encountered among small birds, but the absence of reports may be due to the tendency of investigators to assume conspicuous plumage results from sexual selection, rather than aposematism and distastefulness (251.
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From page 5... ...
Animal phyla in which autogenous chemical defenses are documented Porifera,tt 5000 spp. Sesquiterpenes, sesterterpenes, dibromotyrosine derivatives, isonitriles, polyalkylated indoles, macrolides, quinones, ancepsenolides, sterols Coelenterata:tt Anthozoa Alcyonaria, 9000 spp.
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From page 6... ...
This emphasis may be because the function of secondary chemicals in plants is less immediately apparent to humans, who have historically consumed a broad array of plants without ill effects, so alternative explanations of their presence readily come to mind. The fact that animals upon disturbance often squirt, dribble, spray, or otherwise release noxious substances at humans and cause pain leads to readier acceptance of a defensive function Lalthough there are skeptics who are unconvinced of a TABLE 2 Chemical defense theories Ref.
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From page 7... ...
Plants produce secondary compounds as derivatives of primary metabolism; animals do the same. In fact, plants may be rather unrepresentative of chemical defense strategies as a whole in that they rarely coopt defense compounds from other organisms via sequestration, although there are exceptions to the general rule [e.g., parasitic plants (49-51~1 The relative importance of consumer selection pressure in determining patterns of production of secondary compounds varies with the theory.
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From page 8... ...
Why plants, which have the capacity to make glucose and, from glucose, the storage material starch, should make secondary compounds as "overflow" metabolites is unclear. Since glucose is a starting material for much of secondary metabolism, it is difficult to conceive of how such elaborate pathways could evolve in the absence of any selection pressure other than whatever problems may be associated with fixing too many carbon atoms.
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From page 9... ...
That many theories coexist is at least in part due to the fact that they are not mutually exclusive-they all share certain elements. If there is a recurrent theme in the past century of discussion, it is that chemical defenses confer a benefit and exact a cost.
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From page 10... ...
Current patterns of allocation observed today are the result of an evolutionary process and are likely to change in the future as a result of evolutionary processes; it is difficult to appreciate ecological patterns without at least a rudimentary understanding of their evolutionary underpinnings and understanding the evolutionary process necessitates identifying selective agents and quantifying the selective forces they exert. Restoring evolution to a place of prominence in future discussions of chemical defense means greatly increasing attention to the genetics of chemical defense production and allocation.
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From page 11... ...
There is undeniably some degree of linkage between primary and secondary metabolism-at the very least, dead plants do not manufacture secondary metabolites but there is little evidence to support the linear relationship that is assumed to prevail between them. If secondary metabolism cannot be totally divorced from primary metabolism in studying the ecology and evolution of chemical defense, irrespective of whether it takes place in plants, animals, or any other organisms, it may be productive for a while at least to arrange for a trial separation and see whether paradigms change.
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From page 12... ...
Although there are similarities between humans and other organisms in the acquisition of chemical defenses, there are striking differences in the deployment of these defenses (Table 31. Whereas most organisms use chemical defenses to minimize their own risk of being consumed, humans use chemicals in an offensive fashion, with the express purpose of killing off not only potential consumer organisms but also potential competitors for food or shelter.
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From page 13... ...
The general failure of any single all-encompassing theory to gain acceptance to date may indicate that such a theory might not be a biologically realistic expectation. In lieu of refining theory, focusing attention on the genetic and biochemical mechanisms that underlie chemical defense allocation is likely to provide greater insights
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From page 14... ...
In particular, generalizations derived from understanding such mechanisms in natural systems have immediate applications in altering patterns of human use of natural and synthetic chemicals for pest control. I thank Thomas Eisner and Jerrold Meinwald for asking me to think in broad terms about chemical defenses and for serving as an inspiration to me for the past two decades, and I thank Arthur Zangerl and James Nitao for their comments, insights, and unique good cheer.
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From page 15... ...
(1979) in Herbivores: Their Interaction with Secondary Plant Metabolites, eds.
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From page 16... ...
16 / May R Berenbaum Secondary Plant Metabolites, eds.
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