Local adaptation to an altitudinal gradient: the interplay between constitutive trait variation and phenotypic plasticity in Mimulus laciniatus

Organisms can adapt to environmental heterogeneity through two mechanisms: (1) constitutive expression of population genetic variation or (2) phenotypic plasticity. We investigated whether patterns of plastic and constitutively expressed traits along elevational and latitudinal clines in a North American endemic plant, Mimulus laciniatus, were consistent with local adaptation. We grew inbred lines of M. laciniatus from across the species’ range to measure constitutive and plastic trait expression in several traits previously shown to be involved in adaptation to M. laciniatus’s rocky outcrop microhabitat: flowering time, size-related traits, and leaf shape. We then tested for a relationship between variation in trait expression and population elevation and latitude, and examined correlation in trait expression between plastic and constitutive traits. We found that population variation in constitutive leaf lobing is associated with latitude, and identified multiple traits that show patterns consistent with local adaptation to elevation: critical photoperiod, flowering time, flower size, leaf lobing, and leaf lobing plasticity. These trends occur along multiple geographically independent altitudinal clines indicating that selection is a more likely cause of this pattern than gene flow among nearby populations with similar trait values. We did not find a correlation between constitutive and plastic leaf lobing at the individual genotype level, suggesting that while both traits appear to be adaptive, they operate under independent genetic controls. This indicates that both having more highly lobed leaves and greater leaf shape plasticity are likely adaptive at high elevation within M. laciniatus. Our data strongly suggest that traits known to be under divergent selection between M. laciniatus and close relative Mimulus guttatus are also under locally varying selection within M. laciniatus.