Ontogenetic and Geographic Phytochemical Variation in Mimulus moschatus, a Perennial Monkeyflower

Secondary metabolites are critical to plant defenses and frequently exhibit variation among populations associated with heterogeneous climatic factors and/or herbivore communities. We examined genetic-based geographic and ontogenetic variation in phenylpropanoid glycoside (PPG) concentrations and arsenals in Mimulus moschatus, a perennial monkeyflower native to western North America. We conducted a greenhouse common garden with maternal lines collected from three populations sampled along a latitudinal gradient paralleling spatial sampling from a previous study in the closely related congener, Mimulus guttatus. We assessed PPG arsenals as well as concentrations of individual and total PPGs within leaf tissue from two developmental stages. Populations differed in total concentration and arsenal composition of PPGs, with the populations from intermediate latitudes exhibiting higher concentrations than higher and lower latitude populations. Leaves from earlier developmental stages had higher overall PPG concentrations than leaves from later developmental stages across all populations, suggesting juvenile tissue is better defended than more mature tissue. Spatial patterns in relative PPG concentrations and arsenal composition tightly paralleled those in the closely related M. guttatus across space despite consistently lower total PPG concentrations in M. moschatus. Because we did not measure herbivory and our sample size is limited, these results should be interpreted cautiously. Nonetheless, they provide baseline data for understanding geographic and developmental variation in chemical defenses and generate hypotheses about environmental influences and potential convergence in defenses among closely related species.