ITS rDNA polymorphism among species and hybrids of Potamogeton subgenus Coleogeton (Potamogetonaceae) in north-western Europe

Potamogeton filiformis, P. pectinatus and P. vaginatus are three closely related species that produce sterile
hybrids. These hybrids can be extremely difficult to identify, especially if in a vegetative state. To solve this
problem, a protocol involving restriction fragment length polymorphism analysis of internal transcribed
spacer rDNA is described that allows all three species and their hybrids to be distinguished. Analysis of
internal transcribed spacer sequences shows that P. filiformis is closely related to P. vaginatus (differing by
only a single substitution), whereas P. pectinatus differs from both by three indels and at least 20
substitutions. Infra-specific sequence variation was seen only in P. pectinatus, in which material from arctic
Russia differed from that from Scotland, the Nordic region, the Netherlands and Italy by seven mutations.