A combined study of morphology, stem anatomy and isozyme patterns was used to reveal the identity of sterile plants from two rivers on the Germany/France border. A detailed morphological examination proved that the putative hybrid is clearly intermediate between Potamogeton natans and P. nodosus. The stem anatomy had characteristics of both species. The most compelling evidence came from the isozyme analysis. The additive “hybrid” banding patterns of the six enzyme systems studied indicate inheritance from P. natans and P. nodosus. In contrast, other morphologically similar hybrids were excluded: P. ×gessnacensis (= P. natans × P. polygonifolius) by all the enzyme systems, P. ×fluitans (= P. lucens × P. natans) by AAT, EST and 6PGDH, and P. ×sparganiifolius (= P. gramineus × P. natans) by AAT and EST. All samples of P. ×schreberi are of a single multi-enzyme phenotype, suggesting that they resulted from a single hybridization event and that the present-day distribution of P. ×schreberi along the Saarland/Moselle border was achieved by means of vegetative propagation and long-distance dispersal. Neither of its parental species occur with P. ×schreberi or are present upstream, which suggests that this hybrid has persisted vegetatively for a long time in the absence of its parents. The total distribution of this hybrid is reviewed and a detailed account of the records from Germany is given. P. ×schreberi appears to be a rare hybrid. The risk of incorrect determination resulting from the identification of insufficiently developed or inadequately preserved plant material is discussed.
A Potamogeton hybrid found growing in the absence of parental species in a South Bohemian stream, Czech Republic, was subjected to molecular analyses to identify its exact identity. RFLP of the ITS region confirmed its previous morphological identification as P. natans × P. nodosus (= P. ×schreberi). A comparison of its RFLP pattern with those of P. gramineus, P. lucens and P. polygonifolius unambiguously excluded the possibility that the investigated plants are specimens of other similar hybrids (P. ×fluitans, P. ×sparganiifolius, P. ×gessnacensis). The discovery of P. ×schreberi in South Bohemia is the first record of this hybrid for the Czech Republic. So far, it is known only from five countries and the Czech clone is one of a few extant clones of this hybrid in Central Europe. Chloroplast DNA sequencing identified P. nodosus as the maternal parent although at present this species neither occurs at the locality, nor upstream, nor in the entire drainage basin. The other species, P. natans, only occurs downstream of the locality in isolated side pools in a former stream bed and fishponds in an adjacent drainage basin. The available data indicate that this hybrid has persisted vegetatively at this locality for some time in the absence of its parents.