Bad Taste Protects Fruit Flies from Eating a Toxic Amino Acid in Plants

摘要

Besides enhancing the pleasure of eating, our sense of taste can steer us away from poisonous foods. Many plants, for example, produce bitter-tasting toxins, such as caffeine and quinine, to deter predation by herbivores from cows to insects. But it remains a mystery how animals developed their capacity to detect—and so avoid—the tens of thousands of plant toxins, which include alkaloids, phenolics, and nonprotein amino acids. The best-known toxic nonprotein amino acid is L-canavanine, which accumulates in the seeds of alfalfa (Medicago sativa) and many other leguminosae. L-canavanine is so similar to the amino acid L-arginine that it gets incorporated into proteins, rendering them dysfunctional. A few insects have developed strategies for counteracting this toxin, including larvae of the beetle Caryedes brasiliensis, which depend entirely on seeds of the legume Dioclea megacarpa and catabolize L-canavanine into harmless compounds. However, the vast majority of insects are susceptible to L-canavanine, and little is known about their ability to detect it in plants.