Constituents of a mixed-ploidy population of Solidago altissima differ in plasticity and predicted response to selection under simulated climate change

Premise of the Study: Polyploids possess unique attributes that influence their environmental tolerance and geographic distribution. It is often unknown, however, whether cytotypes within mixed-ploidy populations are also uniquely adapted and differ in their responses to environmental change. Here, we examine whether diploids and hexaploids from a single mixed-ploidy population of Solidago altissima differ in plasticity and potential response to natural selection under conditions simulating climate change. Methods: Clonal replicates of diploid and hexaploid genotypes were grown in a randomized split-plot design under two temperature (+1.9°C) and two watering treatments (–13% soil moisture) implemented with open-top passive chambers placed under rainout shelters. Physiological, phenological, morphological traits, and a fitness correlate, reproductive biomass, were measured and compared among treatments. Key Results: Differences in traits suggest that diploids are currently better adapted to low- water availability than hexaploids. Both ploidy levels had adaptive plastic responses to treatments and are predicted to respond to selection, but often for different traits. Water availability generally had a stronger effect than temperature, but for some traits the effect of water depended on temperature. Conclusions: Diploid and hexaploid S. altissima may maintain fitness in the short term through adaptive plasticity and evolution depending on which traits are important in a warmer, drier environment. Hexaploids may be at a disadvantage compared to diploids because fewer traits were heritable. Our results underscore the importance of studying combinations of climate variables that are predicted to change simultaneously.