RAD sequencing resolved phylogenetic relationships in European shrub willows (Salix L. subg. Chamaetia and subg. Vetrix) and revealed multiple evolution of dwarf shrubs

Abstract

The large and diverse genus Salix L. is of particular interest for decades of biologicalresearch. However, despite the morphological plasticity, the reconstruction of phylogenetic relationships was so far hampered by the lack of informative molecular markers. Infrageneric classification based on morphology separates dwarf shrubs (subg.Chamaetia) and taller shrubs (subg. Vetrix), while previous phylogenetic studies placedspecies of these two subgenera just in one largely unresolved clade. Here we want totest the utility of genomic RAD sequencing markers for resolving relationships at different levels of divergence in Salix. Based on a sampling of 15 European species representing 13 sections of the two subgenera, we used five different RAD sequencingdatasets generated by Ipyrad to conduct phylogenetic analyses. Additionally we reconstructed the evolution of growth form and analyzed the genetic composition ofthe whole clade. The results showed fully resolved trees in both ML and BI analysiswith high statistical support. The two subgenera Chamaetia and Vetrix were recognized as nonmonophyletic, which suggests that they should be merged. Within theVetrix/Chamaetia clade, a division into three major subclades could be observed. Allspecies were confirmed to be monophyletic. Based on our data, arctic-alpine dwarfshrubs evolved four times independently. The structure analysis showed five mainlyuniform genetic clusters which are congruent in sister relationships observed in thephylogenies. Our study confirmed RAD sequencing as a useful genomic tool for thereconstruction of relationships on different taxonomic levels in the genus Salix.