Receptor discrimination of enantiomers of the aggregation pheromone ipsdienol, in two species ofIps

Abstract

The stimulation effect of the two enantiomers of the male-produced aggregation pheromone, ipsdienol, was tested by electrophysiological recordings from single olfactory cells in females of two species,Ips pini andI. paraconfusus. The results demonstrated two types of receptor cells, each specialized to one of the optical configurations. This suggests that separate acceptors (membrane receptors) for (+)- and (−)-ipsdienol are produced by the beetles' receptor cells. The dose-response curves obtained for the 92% “pure” enantiomers and racemic mixtures indicated no synergistic or inhibitory interaction of the enantiomers on the receptor cells. The results could be explained by activation of one acceptor type in each cell group.

I. paraconfusus apparently had the majority of its ipsdienol cells keyed to the (+)-enantiomer. Conversely, the westernI.pini had more cells tuned to (−)- than to (+)-ipsdienol. This difference is consistent with behavioral responses where these species are sympatric in California. The (+)- and (−)-ipsdienol are aggregation pheromone components ofI. paraconfusus anaI. pint, respectively, and the opposite enantiomers act as aggregation inhibiting allomones. More (−)- than (+)-ipsdienol cells were also obtained in the easternI. pint, even though this population produces more (+)- than (−)-ipsdienol (63∶35) and requires both enantiomers for aggregation behavior. However, the difference in the numbers of (+)- and (−)-ipsdienol cells recorded from the eastern population was insufficient for an acceptable level of statistical confidence.