Molecular identification of hybrids from a former hot spot of Potamogeton hybrid diversity

More than a century ago, an extremely high diversity of Potamogeton hybrids was recorded in several rivers and streams in Jutland, Denmark. Accuracy of their identification was doubted by some later authors who were sceptical about the existence and co-occurrence of so many hybrids. Selected original localities were investigated for the presence of morphologically matching plants after more than 100 years. In spite of profound changes in landscape and considerable deterioration of aquatic habitats during the 20th century resulting in a significant decline of submerged vegetation, three of ten previously recorded hybrids were recently found that had persisted at their original localities. Two of them, whose existence had not been proved previously, were subjected to molecular analyses. RFLP, direct sequencing and cloning of the ITS region confirmed their previous morphological identification: P. × undulatus as P. crispus × P. praelongus and P. × cognatus as P. perfoliatus × P. praelongus. Chloroplast DNA sequencing identified P. praelongus as the maternal parent in both crosses. The existence of most of the other hybrid combinations recorded from Jutland was proved by means of molecular analyses conducted on plants from other regions. Their morphologies perfectly correspond to old herbarium vouchers from Jutland and support their original identifications. These observations indicate that Jutland rivers and streams hosted a high species and hybrid diversity still in the late 19th century, but most of this richness has meanwhile disappeared.

► High diversity of Potamogeton hybrids was recorded from Jutland, Denmark in the past.
► These records were doubted by later authors.
► Molecular analyses confirmed identifications made more than a century ago.
► Most of the former high diversity of Jutland streams disappeared in the 20th century.