Defensive compounds

The first volume ends with a chapter by G. Pohnert on chemical defence in the marine environment. Defense compounds, which can be regarded as allomones, are often, but not always, more complex than other semiochemicals and may have unique modes of action. The biological mechanisms are not always easy to unravel, which is shown by some of examples. The reader may be tempted to compare the chemical complexity with that of terrestrial insect defence, which can be found in the second volume chapter by D. Daloze and J.-C. Braekman. Insects thus do not only produce interesting pheromones, but also complex allelochemicals for their own protection. 

Host Marking, Spacing (Epideictic), and Defense Compounds.148... 

Perhaps the most interesting arthropodan defensive compounds from the point of view of structural diversity are the alkaloids. While alkaloids had long been believed to arise only as a consequence of plant secondary metabolism, it has become apparent over the last few decades that arthropods are both prolific and innovative alkaloid chemists. The millipede Polyzonium rosalbum, once thought to secrete camphor (20), in fact gives off a camphoraceous/earthy aroma produced by the spirocyclic isoprenoid imine polyzonimine (21). 

In this discussion, we have restricted ourselves to the consideration of only a few examples of arthropod chemistry. From these alone, it is evident that insects synthesize defensive compounds by using all of the major biosynthetic pathways, producing acetogenins, simple aromatics and quinones, isoprenoids, and alkaloids. In addition, some of the millipedes, coccinellid beetles, and spiders we have studied utilize biosynthetic pathways that have yet to be characterized. 

Finally, Chaps. 13 and 14 both review other areas of macroalgal and microalgal chemical ecology that have been studied to some extent for a number of years but which are both active areas of current research. Chapter 13 focuses on the multiple ways in which algae utilize defensive compounds to limit damage from ultraviolet radiation. Chapter 14 reviews studies of the behavioral sensory ecology of algae, which is very much understudied in comparison to such work on terrestrial and aquatic animals. 

As far as we know, one species, or a few closely related ones, often have a monopoly on a particular compound. The tobacco plant (Nicotiana tabacum), for example, synthesizes nicotine to defend itself from attack by herbivores (animals that feed on plants) and as a convenient way of storing nitrogen temporarily. Neither roses, elephants, nor starfish contain nicotine. In fact, apart from a few of tobacco s close relatives, we know of no other living organism that makes nicotine. In the same way, other plants have devised their own defensive compounds for protection from herbivores. 

Adults of some species also produce 4-oxo-( )-2-alkenals. Other types of simple compounds that have been found in the defensive secretions of true bugs include common terpenoids such as a- and (3-pinenes, limonene, linalool, and Z, -oc-farnesene, and simple aromatic compounds such as benzyl alcohol, ben-zaldehyde,p-hydroxybenzaldehyde, methyl p-hydroxybenzoate,phenylethanol, and guaicol. In general, although a number of species may share particular components, each species does appear to produce its own particular blend. In at least one species, the blend of defensive compounds is reported to vary with season and/or diet [36]. 

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