Hybridisation and cryptic invasion in Najas marina L. (Hydrocharitaceae)?

Macrophytes have been used as bioindicators for eutrophication assessment in freshwaters required by the European Water Framework Directive (WFD). The red listed Najas marina s.l. is routinely mapped in Germany. Different indicator values have been assigned to the subspecies marina and intermedia which are, however, frequently hard to tell apart due to morphological similarity. Therefore, phylogenetic structure within N. marina s.l. was investigated using nuclear ribosomal (ITS) and chloroplast (trnL-F) DNA sequence data from over a hundred accessions, representing three of the 12 subspecies and one of four varieties in N. marina. The samples group in two distinct clusters, which could be correlated to the two karyotypes previously reported. The clusters differ in 45 positions of ITS and 10 of trnL-F, respectively, with almost no variation within. Conflicting placement in the nuclear and chloroplast tree supported by cloning of heterozygotic samples identified hybrids in four cases. The clear-cut molecular differentiation in spite of morphological similarity identifies both lineages as distinct but cryptic species (N. marina and N. major). Based on our modified concept and the uncertainty introduced by former misidentification, the use of the two taxa for the purpose of the WFD and regional red list status needs re-evaluation.