Insights into the Taxonomically Challenging Hexaploid Alpine Shrub Willows of Salix Sections Phylicifoliae and Nigricantes (Salicaceae).

The complex genomic composition of allopolyploid plants leads to morphologically diverse species. The traditional taxonomical treatment of the medium-sized, hexaploid shrub willows distributed in the Alps is difficult based on their variable morphological characters. In this study, RAD sequencing data, infrared-spectroscopy, and morphometric data are used to analyze the phylogenetic relationships of the hexaploid species of the sections and in a phylogenetic framework of 45 Eurasian species. Both sections comprise local endemics as well as widespread species. Based on the molecular data, the described morphological species appeared as monophyletic lineages (except for s.str. and , which are intermingled). Both sections and are polyphyletic. Infrared-spectroscopy mostly confirmed the differentiation of hexaploid alpine species. The morphometric data confirmed the molecular results and supported the inclusion of into s.l., whereas the alpine endemic is distinct and closely related to species of the section . The genomic structure and co-ancestry analyses of the hexaploid species revealed a geographical pattern for widespread , separating the Scandinavian from the alpine populations. The newly described is tetraploid and is grouped within . Our data reveal that both sections and need to be redefined.