Molecular and phytochemical evidence for the taxonomic integrity of Salix alba, S-fragilis, and their hybrid S. x rubens (Salicaceae) in mixed stands in SE Germany

AFLP fingerprinting, nrDNA sequencing, and HPLC of phenolic compounds from bark samples was used to assess the intensity of hybridisation in two mixed stands of Salix alba, S. fragilis, and their hybrid S. x rubens (Salicaceae) along the Danube in the vicinity of Regensburg (SE Germany). AFLP analyses of 130 individuals from two mixed and two pure strands of the parental species resulted in a matrix of 337 polymorphic loci that was analysed with Bayesian cluster algorithms implemented in the software programmes Structure and NewHybrids. The results, together with sequence polymorphisms of the nrDNA ITS1 + 5.8S region, indicate that the three taxa are distinctly separated and that only a few F2-hybrids and no backcrosses to the parental species are present in the surveyed mixed stands. These findings were corroborated by HPLC studies of the phenolic compounds from bark samples that were carried out for the same individuals. A multivariate statistical analysis of relative peak areas of chromatograms (PCA, principal component analysis) indicates the clear separation of parental and the intermediate position of S. x rubens individuals.