The evolution of aggregation pheromone diversity in bark beetles

Bark beetles of the subfamily Scolytinae employ aggregation pheromones to attract conspecifics to their host trees (predominantly conifers) for the purposes of mating and resource exploitation. These aggregation pheromones consist of blends of chemical components whose composition is typically species specific. Across tribes and genera within the Scolytinae there are clear phylogenetic patterns in these compounds. However, within these clades differences in pheromone composition may be as great between closely related species as between more distantly related species.Here we review our understanding of the evolution of aggregation pheromone diversity in this group including discussion of the mode of evolution, and the ecological, genetic and biosynthetic mechanisms that may explain this diversity. We also collate information from the literature on the aggregation pheromone composition for 65 species of bark beetles across 17 genera, including intraspecific variation. We relate this information to evolutionary relationships to ascertain the extent to which pheromone composition reflects phylogeny. As with previous studies, we show that the mode of pheromone evolution can be characterised by substantial saltational changes in chemical composition at speciation events. We also demonstrate that some individual components are strongly phylogenetically conserved within clades, while others are phylogenetically labile. This suggests that differences in aggregation pheromone composition may arise through a combination of relatively rapid 'switching on and switching off' of certain compounds (eg, ipsdienol and ipsenol in the Ipini), while other compounds remain more stable once their use has evolved. We discuss these findings in the context of recent theory on the evolution of species-specific attractive pheromones.