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.