Repeatability of phenotypic consequences due to whole‐genome duplication in <i>Spirodela polyrhiza</i>

Summary Whole‐genome duplication (WGD) is widespread in plants, yet the extent to which it yields predictable phenotypic outcomes remains unclear. Here, we show that the phenotypic consequences of genome doubling in a duckweed model system, Spirodela polyrhiza , are highly repeatable and largely deterministic. We previously generated three independent colchicine‐induced autotetraploids from each of nine globally distributed diploid genotypes and now quantified growth and morphology across a salt gradient. In benign conditions, diploids grew faster, whereas tetraploids had larger, thicker fronds. As salinity increased, the diploid growth advantage diminished, and tetraploids frequently matched or exceeded diploid growth rates. By partitioning the components of variance in growth in our experimental design, we found that ploidy per se explained a comparable amount of phenotypic variation in growth and substantially more variation in salt tolerance than the genotypic background, with evidence of rare within‐genotype stochastic differences between tetraploids. These results indicate that the shifts in morphology and stress tolerance from genome doubling are predictable and can match the phenotypic effect from genetic sequence diversity.