Polyploid—Diploid coexistence in the greater duckweed Spirodela polyrhiza

Polyploidy is widespread among plants and is often associated with stressful environments that can facilitate their establishment. Successful new polyploids must overcome competitive disadvantages against their progenitors or benefit from negative frequency‐dependent growth.

We tested the invasion of neotetraploid duckweed strains ( Spirodela polyrhiza) into populations of their diploid progenitors in control and salt stress conditions in replicated microcosms. We also tested the reverse scenario: invasion of diploids in neotetraploid populations, to examine the impact of the initial tetraploid proportion.

Tetraploid proportions declined in all tetraploid and diploid invasions, with different rates than predicted from only intrinsic growth rate differences. Salt stress affected the decline in a strain‐specific manner. We found evidence for negative frequency‐dependent growth that, nonetheless, was insufficient to overcome competitive disadvantages of neopolyploids.

Synthesis . Our findings demonstrate the crucial role of competition and frequency dependence on neopolyploid establishment. Therefore, we caution against inferring neopolyploid success based solely on intrinsic growth rates.