Polyploid - diploid coexistence in the greater duckweed Spirodela polyrhiza

Polyploidy, resulting from whole-genome duplication (WGD), is widespread among plants and originates in sympatry with their lower-ploidy progenitors. New polyploids can only succeed when they overcome the competitive disadvantage against their progenitors or through sufficient niche differentiation as resulting in a negative frequency dependent growth. Because polyploidy is frequently associated with cold, dry, and saline environments, stress is anticipated to be the key to polyploid success. We examined the invasion of neotetraploid duckweed (Spirodela polyrhiza) strains in populations of their direct diploid progenitors, in control and salt stress conditions in replicated microcosms. We also tested the reverse scenario, that is, the invasion of diploids in neotetraploid populations, to determine if the initial proportion of tetraploids affects the outcome of competition. Our results showed that the proportion of tetraploids declined in all tetraploid and all diploid invasions and it did so at a different rate than expected from only differences in intrinsic growth rate. Salt stress affected the decline in tetraploid proportion differently across strains. We also found evidence for negative frequency dependent growth that, nonetheless, was insufficient to overcome competitive disadvantages of neopolyploids towards their diploid progenitor. We showcase a robust quantitative pipeline from flow cytometry of mixed-ploidy populations to population model fitting. In doing so, we demonstrate the important effect of competition and frequency dependency on neopolyploid establishment. Therefore, we caution for inferring neopolyploid success from intrinsic growth rates alone.