Parallel evolution and convergent physiological tolerance mechanisms of serpentine-tolerant Achillea millefolium (common yarrow; Asteraceae) edaphic ecotypes

Serpentine soils have exceedingly low Ca:Mg mol ratios (<< 1.0), compared to nonserpentine soils.  Ca is a plant-essential macronutrient that is important for cell wall structural integrity.  Serpentine-tolerant plant physiological adaptations to low soil Ca:Mg mol include selective Ca uptake, Mg exclusion, or both.  Although serpentine tolerance and its implications for speciation have been widely studied utilizing congeneric serpentine-tolerant and intolerant species, there has been little study of the potential for the independent origin of serpentine-tolerant ecotypes (ecotypic convergence) within the same species.  In this study, 16 nonserpentine and 48 serpentine Achillea millefolium ecotypes collected from widespread locations in California and other regions in the US, were examined for seedling tolerance to serpentine soil (Ca:Mg mol = 0.2) and shoot and root Ca and Mg uptake when grown under uniform soil nutrient conditions (Ca:Mg mol = 1.0) in order to determine if there is evidence for serpentine ecotypic convergence.  Results revealed at least 6 unique incidences of serpentine ecotypic convergence (shoot Ca:Mg mol 1.0 – 2.0) with strong evidence of directional selection for enhanced Ca uptake and root-to-shoot translocation across all of the serpentine ecotypes.  Serpentine ecotypes were 100% tolerant to serpentine soil and maintained an average shoot Ca:Mg mol ratio of 0.8, whereas nonserpentine ecotypes were only 13% tolerant (abundant root tip necrosis) and had a shoot Ca:Mg mol ratio of 0.5.  This study demonstrated that serpentine-tolerant ecotypes of Achillea millefolium have arisen independently multiple times and that the genetic preadaptation for serpentine tolerance (selective Ca uptake) in the species is fixed.