Chemosensory tuning to a host recognition cue in the facultative specialist larvae of the moth Manduca sexta

Larvae of Manduca sexta are facultative specialists on plants in the family Solanaceae. Larvae reared on solanaceous foliage develop a strong preference for their host; otherwise, they remain polyphagous. The host-specific recognition cue in potato foliage for Manduca larvae is the steroidal glycoside, indioside D. Two pairs of galeal taste sensilla, the lateral and medial sensilla styloconica, are both necessary and sufficient for the feeding preferences of host-restricted larvae. We conducted electrophysiological tip recordings from sensilla of solanaceous or wheat germ diet-reared larvae. For each animal, recordings of the responses to indioside D, glucose, tomatine and KCl were compared. All responses included both phasic and tonic portions. The sensilla styloconica of solanaceous-reared larvae were tuned to indioside D, defined as maintaining a high sensitivity to indioside D, while showing lower sensitivity to other plant compounds. Half of the sensillar neurons of solanaceous-reared larvae were 'tuned' to indioside D, whereas those of wheat germ diet-reared larvae were not. The different responses between the two types of animals were a result of changes of individual receptor cells' responses in the sensilla. Feeding on solanaceous foliage therefore appears to result in a modification of the physiological responses of individual taste receptor cells that causes them to be tuned to the host-recognition cue indioside D. We propose that this tuning is the basis for the host-restricted larvae's strong behavioral preferences for solanaceous foliage.