A survey of duckweed species in southern Italy provided first occurrences of the hybrid Lemna × mediterranea in the wild

Interspecific hybridization and polyploidization are recognized as two main driving forces in plant evolution, shaping genomes and favoring evolutionary novelty and ecological adaptation. Recent studies have demonstrated hybridization within the genus Lemna (Lemnaceae Martinov). Lemna minor has given rise to two interspecific hybrids: Lemna x japonica, recognized as a species since 1980, and the newly discovered L. x mediterranea, identified among germplasm collection clones. L. x mediterranea, a hybrid between L. minor and L. gibba, was hypothesized to correspond to the species L. symmeter, which was invalidly described about 50 years ago in Southern Italy. A sampling campaign identified eight populations of the hybrid, at different sites across the Campania region, in Italy. The isolated specimens were found to be genetically identical by the nuclear marker Tubulin-based polymorphism (TBP), likely belonging to the same original clone (LER-LME) distinct from previously analyzed collection clones, suggesting recurrent hybridization. The natural hybrid clone is triploid, with L. gibba as the plastid donor. Morphology is very similar to L. gibba, although the typical gibbosity of this species becomes evident only upon flower induction. Flowers are protogynous and self-sterile. Populations of both parent species, L. minor and L. gibba, were recovered during the survey, recording a high genetic variability in L. minor. Other Lemnaceae species, Wolffia arrhiza and L. trisulca were also occasionally present. The presence of the invasive species L. minuta seems to be less prevalent with respect to other Italian regions. Synthesis: Five populations of the cryptic hybrid L. x mediterranea were discovered for the first time in the wild in Southern Italy. Clones isolated from these populations, sampled from distinct water bodies over an area of about 4200 Km2, are genetically indistinguishable and likely originated from the same hybridization event. Thanks to high intron polymorphism, TBP provides a straightforward method for genetically identifying sterile clonal lineages and tracking their spatial and temporal distribution. Ecological factors including competition with parental and invasive species, niche and climate change adaptation, stability in time and space are to be investigated.