Phylogenetic-Pheromone Associations Obscured by Stabilising Selection and Natal Tree Effect in a Tree-Killing Bark Beetle.

Insects are highly reliant on chemical cues such as pheromones to facilitate communication and navigation. Some of the roles of pheromones include attracting and finding mates and conspecifics, and in these cases, we expect stabilising selection to dampen within-population pheromone variation. On the other hand, standing pheromone variation may lead to barriers to gene flow and saltational shifts that facilitate divergence and speciation. We investigated the relationships between pheromone variation and genetic variation in the spruce beetle, Dendroctonus rufipennis Kirby, a bark beetle that infests spruce. We found no convincing associations between genetic variation and pheromone variation in the spruce beetle. Instead, our results suggest that stabilising selection has acted to harmonise regional pheromone blends, including those of different sympatric clades, while pheromone blends differ regionally even within the same clade. Individual pheromone variation within regions cannot be attributed to phylogenetics and is instead partly explained by the identity of the natal tree, suggesting an environmental influence of host tree chemistry. Our results show that stabilising selection is not absolute, and that other opposing forces, such as co-evolution and environmental influences, could contribute to within-population variation.