All flowering plants are ancient polyploids, making polyploidy a principal mechanism promoting biodiversity. While many of the genomic and morphological innovations associated with polyploidy have been elucidated, the ecological consequences remain largely unknown. It has been hypothesized that genomic redundancy and diversity in polyploids contribute to broader ecological breadth and greater ability to cope with rapid environmental changes than diploids. To evaluate this hypothesis, we established a large-scale common garden experiment, comprising 12 clones of 269 genotypes from 69 populations of six polyploid and five diploid Fragaria species, grown in three climatic gardens in Oregon that vary in precipitation, temperature and elevation. We address (1) whether polyploids have higher fitness relative to diploids across all environments; if so, (2) whether polyploid fitness advantage is conferred by greater phenotypic plasticity or mean trait values. We estimated fitness as the product of survival rate, vegetative size, and biomass allocation to asexual reproduction. We measured functional traits: specific leaf area, stomatal size and stomatal density, leaf trichome density, carbon isotope discrimination and leaf nitrogen content. Mixed models with nested random effects, controlling for the influence of climatic niche distance and hierarchical data structure, were conducted to assess mechanisms of adaptation.
Compared to diploids, polyploids had significantly higher fitness across all three environments (χ2 = 4.54, 1 df, P < 0.05). Both polyploids and diploids exhibited similar degrees of trait plasticity, e.g., for specific leaf area, stomatal size and stomatal density (all P > 0.05). However, they differed in mean functional trait values, depending upon the environment. We assessed the contribution of functional trait mean and plasticity to fitness, and found significant positive effects of mean specific leaf area (P < 0.01) and stomatal size (P < 0.01), but not stomatal density, giving polyploids an advantage across all environments. Interestingly, trait plasticity was mostly adaptive for both polyploids and diploids, except stomatal density plasticity being maladaptive for diploids in the most stressful environment. Overall, our results suggest polyploids have a fitness advantage across environments, which is gained through both trait mean and adaptive plasticity. This study provides much-needed evidence for understanding how polyploidy contributes to functional biodiversity, and has important implications for predicting responses to rapidly changing environments.