Evolution of Receptor Sensitivity to Secondary Plant Substances with Special Reference to Deterrents

The evolution of secondary plant substances is generally believed to be related to their role in protecting plants from ingestion by herbivores. The choice of food-plants is based partly on the presence of compounds that act as deterrents. Unless herbivores possess receptors capable of sensing secondary compounds, the central nervous system cannot exercise discrimination, and deterrence is impossible. It seems illogical that receptors would evolve special sensitivity to compounds that would prevent feeding and effectively restrict the range of foodplants, especially since deterrence and toxicity are not invariably coupled. A behavioral and electrophysiological comparison of the gustatory receptors of a nonherbivorous and herbivorous insects reveals that nonherbivorous species have greatly restricted receptor sensitivity as compared with herbivores. It is proposed that instead of evolving specific deterrent receptors, herbivorous insects evolved, from a common chemical sense, receptors sensitive to a very wide variety of compounds. This provided the central nervous system with an expanded pool of information which formed the basis for the evolution at that level of specific deterrent and acceptance behaviors.