Balanced olfactory antagonism as a concept for understanding evolutionary shifts in moth sex pheromone blends

Abstract

In the sex pheromone communication systems of moths, both heterospecific sex pheromone components and individual conspecific pheromone components may act as behavioral antagonists when they are emitted at excessive rates and ratios. In such cases, the resulting blend composition does not comprise the sex pheromone of a given species. That is, unless these compounds are emitted at optimal rates and ratios with other compounds, they act as behavioral antagonists. Thus, the array of blend compositions that are attractive to males is centered around the characterized female-produced sex pheromone blend of a species. I suggest here that the resulting optimal attraction of males to a sex pheromone is the result of olfactory antagonistic balance, compared to the would-be olfactory antagonistic imbalance imparted by behaviorally active compounds when they are emitted individually or in other off-ratio blends. Such balanced olfactory antagonism might be produced in any number of ways in olfactory pathways, one of which would be mutual, gamma-aminobutyric-acid-related disinhibition by local interneurons in neighboring glomeruli that receive excitatory inputs from pheromone-stimulated olfactory receptor neurons. Such mutual disinhibition would facilitate greater excitatory transmission to higher centers by projection interneurons arborizing in those glomeruli. I propose that in studies of moth sex pheromone olfaction, we should no longer artificially compartmentalize the olfactory effects of heterospecific behavioral antagonists into a special category distinct from olfaction involving conspecific sex pheromone components. Indeed, continuing to impose such a delineation among these compounds may retard advances in understanding how moth olfactory systems can evolve to allow males to exhibit correct behavioral responses (that is, attraction) to novel sex-pheromone-related compositions emitted by females.