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. 2026 Jan;35(1):e70226.
doi: 10.1111/mec.70226.

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

R L Isitt  1 , J A Addison  2 , S B Heard  2 , D S Pureswaran  1
Affiliations

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

R L Isitt et al. Mol Ecol. 2026 Jan.

Abstract

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.

Keywords: host chemistry; pheromones; phylogeny; saltational shifts; stabilising selection.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

FIGURE 1
FIGURE 1
Blend ratios of aggregation pheromone components produced by 262 female spruce beetles, split into five clusters (pheromone profiles) by k‐means clustering. Pheromone profiles were named by concatenating the first letter of each non‐zero (by median) pheromone component in order of decreasing median.
FIGURE 2
FIGURE 2
Maximum‐likelihood phylogeny of unique haplotypes of spruce beetle ( Dendroctonus rufipennis Kirby) from a 1114 bp region of mtCOI. Our study resulted in 124 unique haplotypes, with 60 additional haplotypes (including the Dendroctonus murrayanae Hopkins outgroup) obtained from Maroja et al. (2007). Note the break in the outgroup branch, which was shortened for clarity.
FIGURE 3
FIGURE 3
Assemblages of pheromone profiles from female spruce beetles ( Dendroctonus rufipennis ) split by genetic clade (based on mtDNA COI sequences) and geographic origin. Empty cells indicate that no beetles were collected in the given combinations of clade and origin. Numbers indicate the count of beetles that produced the indicated pheromone profile.
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