Genetic variation in the constitutive defensive metabolome and its inducibility are geographically structured and largely determined by demographic processes in maritime pine

Abstract

Interspecific phenotypic variation in plant secondary metabolites (PSM) is often explained by biotic and abiotic factors. However, patterns of variation within species do not clearly fit the theoretical predictions. Exploring how genetics, environment and demographic processes shape such variation among and within populations is crucial for understanding evolution of PSM, particularly in long‐lived plants such as forest trees. Here, we quantified genetic variation in PSM among and within populations, and explored drivers of local adaptation by studying the role of climate as a source of population differentiation in PSM of maritime pine. Constitutive profile and concentrations of 63 PSM and their herbivory‐associated inducibility were determined in the bark of 130 clonally replicated genotypes with known familial structure from 10 populations covering the distribution range of the species. We compared neutral and quantitative population genetic differentiation of PSM (FST and QST). Also, we accounted for population genetic structure and kinship among individuals when exploring climate–trait relationships. We found large population differentiation and additive genetic variation in constitutive PSM. Many PSM were inducible, although very low genetic variation was observed with respect to their inducibility. QST–FST comparisons suggest that differentiation of most diterpenes, monoterpenes, and phenolics can be explained by neutral demographic processes. Spatially heterogeneous selection across populations leading to local adaptation was only found for total constitutive sesquiterpenes and a few individual PSM. After accounting for population genetic structure, only the constitutive concentration of two sesquiterpenes showing signs of diversifying selection was predicted by climate, with decreasing concentrations along a growth‐prone climatic gradient. Synthesis. Evolutionary patterns of plant secondary metabolites depended on their chemical nature, with neutral differentiation governing most plant secondary metabolites. Evidence of local adaptation was only found for total constitutive sesquiterpenes and a few individual plant secondary metabolites. The low genetic variation in the inducibility of plant secondary metabolites suggests a conserved model of defensive induction in this species. Since population differentiation linked to past demographic history could lead to false positives of adaptive differentiation signals, accounting for the genetic relatedness among populations is required to infer the environmental determinants of intraspecific genetic variation in putatively adaptive traits such as plant defences.