Reinterpretation of Potamogeton ×nerviger : solving a taxonomic puzzle after two centuries

Hybrids form an important component of Potamogeton diversity but their exact taxonomic identities and distributions are often insufficiently known. Potamogeton nerviger was described from Lithuania in 1827 as a proper species. Based on morphological and anatomical characters, its interpretation has since varied, ranging from synonymization with other species to identification as different hybrids and intraspecific taxa. Currently, it is universally recognized as the hybrid P. alpinus × P. lucens. Using a combined molecular, morphological and anatomical investigation we re-examined the identity of P. ×nerviger, based on both original and recent plant material. We report a successful amplification and sequencing of nuclear ribosomal ITS1 region from a 188year-old type collection. This was shown to be genetically identical to the morphologically matching plants recently collected at the type locality. Comparison with molecular characters of the possible parental species shows that P. ×nerviger is not P. alpinus × P. lucens, as currently believed, but another hybrid, P. nodosus × P. perfoliatus, which is currently called P. ×assidens. This molecular identification is also supported by anatomical evidence. In contrast, the actual existence of the hybrid P. alpinus × P. lucens is doubtful. Consequences for nomenclature and identities of records reported from other sites are discussed.