Odorant Reception in Insects: Functional and Evolutionary Perspectives

Fabre's nineteenth-century observation that smell is central to insect communication spurred entomologists and, later, chemical ecologists, neurobiologists, geneticists, structural biologists, and evolutionary biologists to investigate how insects detect survival-related compounds. Structural biologists resolved the three-dimensional structures of pheromone-binding proteins and odorant receptors (ORs), revealing features that enable specific interactions with semiochemicals. Researchers proposed that ORs evolved from gustatory receptors as insects adapted to terrestrial life and then specialized to detect species-specific sex pheromones. Most insects use both broadly and finely tuned receptors, but migratory locusts rely mainly on finely tuned ones. To test hypotheses, genes were silenced, expressed in empty neurons, or resurrected, leading to receptor de-orphanization and discovery of new semiochemicals through reverse chemical ecology. These receptors and coreceptors are expressed in olfactory receptor neurons (ORNs) within sensilla of the antennae and maxillary palps. Recent evidence suggests ORNs may express multiple receptor types, including odorant, ionotropic, and gustatory receptors.