The monoterpene landscape of North American pines has implications for mountain pine beetle host transition

Conifer monoterpenes are key chemical defences against bark beetles and their fungal symbionts, but their variation across host species, populations, and environments remains incompletely understood. We analyzed phloem monoterpene concentrations and composition from four pine species used by the mountain pine beetle across 11 sites in western North America and examined how host species, geography, environment, and monoterpene synthase (mono-TPS) gene variation relate to differentiation among host monoterpenes. Total monoterpene concentration and composition differed among species, with Pinus albicaulis Engelm. showing the highest concentrations and P. banksiana Lamb. the lowest. However, relative monoterpene composition was not strictly species-specific. Compositional analyses and a monoterpene similarity network revealed similarities among populations of different species, while individual-tree comparisons identified partial overlap in relative monoterpene composition. Variation within species further broadened the observed range of monoterpenes, and regression analyses identified associations between monoterpene concentrations and tree size, latitude, longitude, elevation, summer temperature, and rainfall. Mono-TPS homologs were highly conserved across pine transcriptomes but showed geographic sequence structure, providing evolutionary context for observed chemical differentiation. Together, these results suggest a structured but partially continuous monoterpene landscape among the pine populations. Such compositional similarities may influence the chemical novelty encountered by bark beetles and their fungal symbionts during host colonization.