Ancient polyploidy
The ancestor of all duckweeds have undergone two whole-genome duplications around 95 million years ago.
Recent polyploidy and hybridization
In 1975, Urbanska-Wortykiewicz reported extensive cytological variation within the Lemna genus with a wide range of chromosome numbers within species, suggesting the occurrence of polyploidy. In 1980, Urbanska-Wortykiewicz extended her investigation to the whole Lemnaceae family and confirmed this cytological variation in the other duckweed genera.
More recently, several findings suggest that both polyploidy and hybridization play a prominent role in duckweed evolution. The first in-depth investigation of interspecific hybridization was performed by Braglia et al. (2021). They proposed that Lemna japonica is actually not a separate species, as previously thought, but rather an interspecific hybrid between Lemna turionifera and Lemna minor. Ernst et al. (2023) convincingly confirmed this hybrid nature by assembling multiple genomes, and as such identified both diploid and triploid hybrids. To indicate their hybrid nature, the name Lemna japonica should no longer be used and should instead be replaced by Lemna × japonica. Similarily, interspecific hybridization between Lemna minor and Lemna gibba has recently been reported, referred to as Lemna × mediterranea (see Braglia et al., 2021 and Braglia et al., 2024).