Biological bulletin virtual symposium: the neuroecology of chemical defense.

In 2006, The Biological Bulletin initiated the idea of publishing “virtual symposia,” a series of papers, in a single issue, on a compelling theme that is central to the scope of this journal. The inaugural virtual symposium, on marine invertebrate models of learning and memory, was introduced in an article by McPhie and Miller (2006). Here, we present the second chapter, focusing on “The Neuroecology of Chemical Defense.” The present symposium was inspired by recent discoveries in chemical ecology, which itself is derived from multiple infusions of new ideas over the last half century. Studies in chemical ecology share the common trait of using chemistry to explain biological interactions at the level of individual organisms, populations, or communities. Chemical ecology currently has at least two major, independent branches. The first, chemical communication, developed through pioneering investigations by notable ethologists. For example, Karl von Frisch (1941) described the behavioral reactions of European minnows to alarm substances from skin from damaged conspecifics. These compounds are emitted only after successful predatory attack and thus serve as an early warning to other, yet-unharmed, minnows. Such studies ultimately inspired physiologists to seek to understand the cellular basis of behavior, giving rise to the field of neuroethology. Today, investigations linking chemistry, physiology, and behavior flourish, with a common goal of understanding the molecular basis and evolutionary ecology of chemical communication systems. The second branch of chemical ecology, allelopathics and defense, arose as a consequence of insights by organic chemists. Working mainly on plants, they discovered a variety of unique natural products that lack primary metabolic function. Some of these “secondary” metabolites occur at extraordinarily high concentrations and are synthesized only at considerable energy investment. This interesting paradox was solved using a combination of elegant laboratory and field investigations. Secondary metabolites were found to act as critical determinants of allelopathy, by inhibiting growth or survivorship of competitive plant species, and of chemical defense, by reducing herbivory. In addition to incorporating the idea of defense and allelopathics into general theories on plant ecology at population and community levels, contemporary research in chemical ecology involves topics such as mechanisms of genetic regulation of biosynthesis, models of the acquisition and sequestration of toxins, and contributions of constitutive versus inducible defenses. Over time, the two branches of chemical ecology have merged into a single, unified trunk. No longer is there a clear distinction between bioactive molecules acting either in communication or in defense. Many herbivorous animal species, for example, exhibit resistance, borrowing plant toxins for their own defense and using these same compounds or their metabolites in courtship and mate attraction. Moreover, toxins act as feeding attractants for consumer species that are resistant to its adverse effects. Similarly, plants use volatile chemical constituents for communicating with each other at a distance. Airborne substances released in response to herbivore attack are carried downwind, reaching plants not yet in contact with the predator and activating inducible chemical defenses in their tissues. Thus, lines are blurred between experimental studies on animal and plant taxa, and between chemical communication and defense. Received 24 August 2007; accepted 24 August 2007. * To whom correspondence should be addressed. E-mail: z@biology. ucla.edu; [email protected] Reference: Biol. Bull. 213: 205–207. (December 2007) © 2007 Marine Biological Laboratory