Characterization of the cryptic interspecific hybrid Lemna x mediterranea by an integrated approach provides new insights into duckweed diversity

Lemnaceae taxonomy is challenged by the particular morphology of these tiny free-floating angiosperms, reduced to a single leaf-like structure called frond, without or with one to few roots. Although molecular taxonomy has helped clarify the phylogenetic history of this family, inconsistency between morphological data and nuclear and plastid markers still poses challenging questions in some cases, leading to frequent misclassifications in the genus Lemna. Recently, the finding that Lemna japonica is an interspecific hybrid between Lemna minor and Lemna turionifera, provided a clear explanation to one of such taxonomic questions. Here we demonstrated that L. minor is also capable to hybridize with Lemna gibba, generating a cryptic, previously unrecognized, but widespread taxon in the Mediterranean area. The nothotaxon Lemna x mediterranea is described through the detailed investigation of seven hybrid clones from a living germplasm collection and compared with clones of the putative parental species L. minor and L. gibba. Genetic analysis revealed that two different cytotypes, diploid and triploid, originated by at least two independent hybridization events. Despite high overall similarity, morphometrical, physiological and biochemical analyses showed an intermediate position of L. x mediterranea between its parental species in most qualitative and quantitative characters, and also separation of the two hybrid cytotypes by some criteria. These data provide evidence that hybridization and polyploidization, driving forces of terrestrial plant evolution, contribute to the duckweed genetic diversity and may have also shaped the phylogenetic history of these mainly asexual, aquatic plants. Further elucidation of hybridization mechanisms and flowering regulation will provide perspectives for future breeding strategies.