LAUSR

Abstract Hybridization is common among plants and has been crucial in the evolution of many aquatic plant groups. However, hybrid individuals are often difficult to identify, particularly in the marine environment. We aim to confirm a hybrid origin between two seagrass species for a morphologically-intermediate plant observed in shallow subtidal environments near Perth, Western Australia. Both morphological measures and DNA markers demonstrated a clear case for hybridization among two sympatric Posidonia species from different species complexes: Posidonia australis and P. coriacea. All five samples with the intermediate morphology had the same multilocus genotype (MLG), providing evidence that it resulted from a single hybridization event, followed by local clonal (rhizome) expansion. This result highlights weak reproductive barriers within the genus Posidonia, a trait common among other seagrass genera with sympatric ranges, apparently overlapping morphologies and low phylogenetic divergence.