Identity and relationships of Sempervivum tectorum (Crassulaceae) in the Rhine Gorge area.
Sempervivum tectorum (Crassulaceae), an orophyte widespread in the European high mountains, also grows in rocky habitats of the Rhine Gorge area (Upper Middle Rhine, Mosel and Ahr river valleys). On the background of its long history of cultivation, it is unclear whether S. tectorum is native or naturalized in the Rhine Gorge area. Using 52 accessions of S. tectorum from across its geographical range (except SE Europe) as well as 15 samples of S. calcareum and S. marmoreum in our final sample, we conducted a genotyping-by-sequencing analysis. The genetic data were used for the identification of genetic groups and for the reconstruction of phylogenetic relationships. We found that the material from the Rhine Gorge area forms a homogeneous genetic group that is clearly distinct from populations from the Massif Central, Alps and Pyrenees. The Rhine Gorge material appears to be most closely related to material from the Massif Central. We hypothesize that the disjunct distribution of the Rhine Gorge/Massif Central clade is best interpreted as the remnant of a formerly wider distribution area obtained in a Quaternary glacial. The possibility of recognizing Rhine Gorge material taxonomically is discussed and rejected for morphological and nomenclatural reasons.