Erroneous identities of Potamogeton hybrids corrected by molecular analysis of plants from type clones

Hybridization is one of the main sources of taxonomic complexity in Potamogeton . In spite of long and intensive research, the total hybrid diversity and distribution of many hybrids are still insufficiently known. Identities of two currently recognized hybrids were tested using molecular analyses of plants from the original clones growing at the type localities of their correct names. Additive characters in direct sequencing of the ITS region and the placement of cloned sequences in a Bayesian consensus tree were used to identify the exact identity of these hybrids. Potamogeton ×lanceolatifolius is widely recognized as a hybrid P. gramineus × P. polygonifolius . However, the molecular analyses and cytological investigation identified this Swedish hybrid as P. gramineus × P. nodosus although P. nodosus currently does not occur in the whole of Scandinavia. This hybrid provides some insight into the former distribution area of P. nodosus as affected by past climatic changes. The recently detected P. ×vepsicus was originally described as P. alpinus × P. natans but the molecular investigation revealed that its correct identity is P. natans × P. praelongus . In both cases, the actual identities could hardly have been unequivocally deduced from the morphology of the hybrids. The existence of these hybrid combinations is here confirmed for the first time. The maternally inherited chloroplast rpl20-5'rps12 sequences were used to identify the female parental species. The name “ P . × argutulus ”, previously, but erroneously, used for a hybrid P. gramineus × P. nodosus , is typified and reduced to the synonymy of P. gramineus .