Specificity and redundancy in the olfactory system of <i>Ips typographus</i>: Single-cell responses to ecologically relevant odors.

Abstract:
We screened 150 olfactory sensilla in single-sensillum recordings to unravel the mechanisms underlying

host selection in the spruce bark beetle, Ips typographus (Coleoptera: Curculionidae: Scolytinae). Odor

stimuli comprised of pheromone (various bark beetle spp.), host, and non-host compounds elicited

strong and selective responses from 106 olfactory receptor neurons (ORNs), 45 of which were tuned to

pheromone compounds, 37 to host compounds, and 24 to non-host volatiles (NHV). In addition, 26 ORNs

responded only weakly to any odor stimulus. Strongly responding ORNs were classified into 17 classes.

Seven classes responded primarily to the Ips pheromone compounds: cis-verbenol, ipsenol, ipsdienol

(two classes), 2-methyl-3-buten-2-ol, amitinol, or verbenone, respectively. Six classes responded to the

host compounds: a-pinene, myrcene, p-cymene, myrcene and p-cymene, 1,8-cineole, or D3-carene,

respectively. Four classes responded to NHV: 3-octanol, 1-octen-3-ol, trans-conophthorin, or

indiscriminately to the repellent green leaf volatiles (GLVs) 1-hexanol, Z3-hexen-1-ol and E2-hexen-

1-ol, respectively. Indiscriminate responses from GLV neurons might explain a behavioral redundancy

among these GLVs. This is the first description of individual bark beetle ORNs dedicated to NHV

perception. These comprise almost 25% of the strongly responding neurons, demonstrating that a large

proportion of the olfactory system is devoted to signals from plants that the insect avoids.