Patterns of variation and correlation in the monoterpene composition of xylem oleoresin within populations of ponderosa pine

Abstract

Monoterpenes are important defensive chemicals that may experience diversifying selection from diverse herbivore assemblages. It is thus important to understand what trade-offs exist in the naturally occurring variation in monoterpene profiles. We measured the absolute concentration (mg/mL) of nine monoterpenes in the xylem resin of ponderosa pine from two populations. The size of the total monoterpene pool exhibited continuous (unimodal) variation. α-pinene, β-pinene and Δ-3-carene account for the bulk of this pool, and in both populations, these three monoterpenes show a discontinuous pattern of variation, such that each individual tree could be assigned to one of five characteristic chemotypes. Moreover, the absolute concentrations of these monoterpene fractions are negatively correlated across individuals in both populations, such that an increase in the concentration of any one is correlated with a decrease in the total pool of all the remaining monoterpenes. The use of absolute concentrations rather than relative amounts demonstrates that such correlations are not a statistical artifact of compositional data. Taken together, these observations suggest that while the size of the total monoterpene pool is constrained, and is likely influenced by many genes and environmental factors, naturally occurring variation in the allocation of this pool to competing monoterpene fractions is controlled by relatively few genes. Such allocational trade-offs may place a constraint on the response of populations to selection by multiple herbivore species