Neuroethological Basis to Evolution of New Olfactory Communication Systems

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Mate finding in most moth species involves long-distance signaling via female-emitted sex pheromones, which have been shown to include a great diversity of chemical structures, even amongst closely related species. The critical importance of these communication signals in the mate recognition system of a species supports the idea that the evolution of the species-specific signals is involved in the speciation process. The conundrum, however, is how signal divergence has occurred because pheromone communication systems appear to possess strong resistance to any changes in the pheromone blends. A surprising discovery of an inactive gene in the sex pheromone gland of female European corn borers that, if it were activated, could be used to produce different pheromone compounds, has shed some light on how major shifts can occur in pheromone production. This gene is not only present, but is active, in the pheromone gland of a closely related and recently (ca. 1 million yrs. ago) evolved species, Asian corn borer, and creates the different blend used by Asian corn borer females. Of equal interest is that 3% of the male European corn borers (rare males) in a laboratory population possess the ability to be attracted to not only to their own females' pheromone components, but also to the much different ones emitted by Asian corn borer females. These broadly responding males might be exactly the type in an ancestral population that could slowly, over generations, track females emitting the new pheromone blend and then develop a preference for the new blend. The shift to preferring the new blend might also include development of inhibition to some of the previous compounds to shut down the attraction of these males to the original ancestral pheromone. This would provide the basis for males from the ancestral population to track slowly over to the new pheromone blend, possibly with some antagonism developing for the original ancestral pheromone blend. In order to understand how male moths can shift their pheromone response from one blend to that of another composed of different compounds requires a comparative study of normal and rare European corn borer males and also Asian corn borer males. The study will include behavioral and electrophysiological responses to attractive pheromone components and also compounds inhibiting attraction of these males to characterize olfactory receptor differences that would allow the shift to occur. It is anticipated that changes in specificity and agonist/antagonist properties of existing receptors in the European corn borer have occurred to generate receptor profiles found in Asian corn borer males. Additional studies will be conducted to test the hypothesis that a major gene controls the rare male phenotype, and that the population frequency of the phenotype can be increased through selection. All the data above should help to shed light on how pheromone blends can change, and, in turn, influence the evolution of new species.