Highly repeatable phenotypic consequences of whole-genome duplication in Spirodela polyrhiza

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, here, quantified growth and morphology across a salt gradient. In benign conditions, diploids grew faster, whereas tetraploids had larger, thicker fronds; with increasing salinity, the diploid advantage diminished and tetraploids often matched or exceeded diploid growth. Variance partitioning revealed that ploidy per se explains more phenotypic variation, including salt tolerance, than genotypic background, with rare within-genotype stochasticity. These results indicate that the shifts in morphology and stress tolerance from genome doubling are predictable and can outweigh the phenotypic effect from genetic sequence diversity.