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Competitive asymmetry confers polyploid advantage under environmental stress

Competitive asymmetry across heterogeneous environments is crucial for the success of polyploid plants, however, little is known about it. As the major force in plant evolution, polyploids are predicted to maintain the competitive dominance relative to diploids under increased stress conditions. To evaluate the hypothesis of competitive asymmetry, we competed tetraploid and diploid plants of perennial herbaceous Chrysanthemum indicum L. (Asteraceae) at different relative frequencies under low and high water stress. We quantified the interaction intensity between competing plants of the same (intraploidy) and different ploidy levels (interploidy), and measured functional traits related to gas exchange and plant water use to understand the underlying mechanisms. Stronger competitive effects of the tetraploid on diploid provided evidence for the competitive asymmetry between polyploid and diploid plants in C. indicum. Such competitive asymmetry was not only maintained under drought (increased water stress), but also translated into higher fitness of the tetraploid consistently across water stress conditions. Functional traits associated with fast growth and efficient water use likely explained the competitive dominance of the tetraploid. These results will advance our understanding of species interactions between polyploid and diploid plants, and provide insights into population dynamics and species distribution under environmental change.