Molecular evidence for two rare Potamogeton natans hybrids with reassessment of Potamogeton hybrid diversity in Poland

The paper provides a novel molecular demonstration of the hybrid origin of two rare Potamogeton natans hybrids, namely P. × gessnacensis (=P. natans × P. polygonifolius) and P. × vepsicus (=P. natans × P. praelongus), based on nuclear and plastid DNA sequence data. The results of the DNA-based survey were somehow unexpected due to incongruence with the primary background of a preliminary morphological identification of the putative hybrid collections. Our results further demonstrate the incertainty in treatment of hybrids in several taxa of Potamogeton based on morphological characteristics. In this context, we suggest that morphological characters and chorological data of at least a part of P. natans hybrids, particularly P. × sparganiifolius (=P. gramineus × P. natans) and P. × vepsicus, should be verified. Taking into account the new data, other recently published records and rectification of erroneous reports, we also present a reappraisal of the current knowledge on Potamogeton hybrid diversity in Poland as compared with selected other European areas well studied in this respect (British Isles, Czech Republic, Germany), with a notion on general rules shaping the distribution pattern of these taxa in Europe. According to the current data, 12 Potamogeton hybrids have been recorded so far in Poland. Among 32 Potamogeton hybrids reported at least in one of the aforementioned areas, Potamogeton × angustifolius, P. × nitens and P. × salicifolius are the most common taxa, while 18 hybrids are based on records from a single country and predominantly from few localities only. Sparse records of at least part of the rare hybrid taxa reflect a still insufficient state of knowledge on the regional hybrid diversity. However, both at the scale of Poland and Europe, the likely general pattern of the occurrence of Potamogeton hybrids in Europe may indicate that their diversity and frequency of occurrence increase with latitude.