Most studies of signal evolution have concentrated on auditory and visual signals. In some cases, studies have shown that receivers are able to respond to a wider range of signals than are commonly produced by conspecifics. A new study by Wendell Roelofs and colleagues demonstrates that a similar phenomenon can occur in chemical signaling between moths.
Most insects rely on their sense of smell for sexual communication. Female moths emit a species-specific pheromone, either a single chemical or a blend of structurally related chemical compounds. The pheromone is produced by desaturase enzymes from unsaturated fatty acid precursors in an abdominal pheromone gland. Male antennae are finely tuned to the female pheromone. Males are able to identify their species-specific components and ratios and can follow small quantities of airborne pheromone to a calling female.
Pheromones have been identified in five species of Ostrinia moths. Four use a blend of Z and E11-14 acetates, but the Asian corn borer, O. furnacalis, uses a blend of Z and E12-14 acetates. Roelofs et al. argue that both female production of and male response to the pheromone have changed in O. furnacalis, relative to the European corn borer, O. nubilalis.
The authors found genes in both Ostrinia species for the Δ11 desaturase, which produces the O. nubilalis pheromone, and theΔ14 desaturase, which produces the O. furnacalis pheromone. While the Δ14 desaturase is non-functional in O. nubilalis, theΔ11 desaturase is non-functional in O. furnacalis. The biochemical pathways for pheromone production indicate that switching fromΔ11 desaturase to Δ14 desaturase produced the O. furnacalis blend rather than the O. nubilalis blend. If O. furnacalis arose from a species with a pheromone consisting of Z and E11-14 acetates, then evolution of the new pheromone blend is likely to have proceeded by duplication, mutation and a change in expression of desaturase genes.
But what about the males? Female moth pheromone production and male response are not genetically linked, so without a male around who liked what he smelled, the female ancestors of O. furnacalis that produced the new pheromone blend would have quickly died out. In wind-tunnel experiments,Roelofs and his team found that a large proportion (4%) of male O. nubilalis were attracted to both their species-typical pheromone blend and that of O. furnacalis. This suggests that when the O. furnacalis pheromone blend appeared, males could have responded to the new female pheromone, allowing speciation to occur while preserving the sender—receiver coupling essential for chemical signaling.
The research of Roelofs et al. presents one of the first examples of mechanisms by which evolution of a pheromone communication system and subsequent speciation may occur. This research raises practical issues for the practice of mating disruption, a pest control technique in which large amounts of insect pheromone are dispensed in agricultural environments, resulting in reduced larval damage to crops by disrupting adult sexual communication. Many pesticides have lost their effectiveness due to evolution of resistance in target insect species. Although resistance to mating disruption has never been observed, the findings of Roelofs et al. suggest mechanisms whereby it could occur.