A survey of duckweed species in Southern Italy provided first distribution records of the hybrid Lemna × mediterranea in nature

Interspecific hybridisation and polyploidization are two main driving forces in plant evolution, shaping genomes and favouring evolutionary novelty and ecological adaptation. Recent studies have demonstrated hybridisation within the genus Lemna (Lemnaceae Martinov) as well as triploid accessions. Lemna × mediterranea, a recently described hybrid between Lemna minor and Lemna gibba, was identified only among long-lasting germplasm collections of in vitro propagated plants, originally collected at different times in the Mediterranean area. We report the first distribution record of L. × mediterranea in the nature, in the Campania region of Southern Italy, the same area where Lemna symmeter was described as a new species about 50 years ago, confirming their synonymy. Eight specimens isolated from five different sampling sites over an area of about 4200 km² showed identical genetic profiles by Tubulin-Based Polymorphism (TBP) analysis, suggesting their common origin from the same hybridisation event, followed by clonal dispersal. The L. × mediterranea population of Campania is genetically different from any of the previously analysed clones, suggesting that recurrent hybridisation between the parental species may occur. The natural hybrid clone is triploid, with L. gibba as the plastid donor, and remarkably similar to it by morphology, although the typical gibbosity of this species becomes evident only upon in vitro flower induction. Flowers are protogynous and self-sterile. Ecological factors including competition with parental and invasive species, niche and climate change adaptation, stability in time and space likely played a role in the successful establishment of L. × mediterranea.