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

Abstract

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 Nigricantes and Phylicifoliae in a phylogenetic framework of 45 Eurasian Salix 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. phylicifolia s.str. and S. bicolor, which are intermingled). Both sections Phylicifoliae and Nigricantes are polyphyletic. Infrared-spectroscopy mostly confirmed the differentiation of hexaploid alpine species. The morphometric data confirmed the molecular results and supported the inclusion of S. bicolor into S. phylicifolia s.l., whereas the alpine endemic S. hegetschweileri is distinct and closely related to species of the section Nigricantes. The genomic structure and co-ancestry analyses of the hexaploid species revealed a geographical pattern for widespread S. myrsinifolia, separating the Scandinavian from the alpine populations. The newly described S. kaptarae is tetraploid and is grouped within S. cinerea. Our data reveal that both sections Phylicifoliae and Nigricantes need to be redefined.

The role of hybrid zones on the phylogeography of two willow species pairs (Salix L.) in the European Alps

“…turned out to be confused with S. phylicifolia L. (Fig. 1B; see also [35]). Both species are medium-sized shrubs and occur in the subalpine zone.…”

Challenge accepted: Evolutionary lineages versus taxonomic classification of North American shrub willows (Salix)

“…The observed diversity in ploidy levels in North American willows, sometimes within species, has also been shown previously (Argus, 2010). The slightly lower number of polyploids in the EuA clade is biased by our reduced sampling for this clade with a focus on diploids (but see Wagner et al, 2020Wagner et al, , 2023. Presumably, 40% of willow species are polyploid (Suda and Argus, 1968) and have a base chromosome number of x = 19 (Argus, 1997).…”

“…In a recent Populus L. and Salix study employing a target capture data set on a broad sampling, Sanderson et al (2023) revealed that high amounts of ancestral hybridization resulted in conflicting phylogenetic signals, especially in genus Salix. RAD sequencing (Baird et al, 2008) was recently used to overcome the lack of phylogenetic information within the Chamaetia/Vetrix clade (Gramlich et al, 2018;He et al, 2021;Wagner et al, 2018Wagner et al, , 2020Wagner et al, , 2023. Using this method, Wagner et al (2018) published the first well-resolved phylogeny of diploid European members of the Chamaetia/Vetrix clade.…”

“…Using this method, Wagner et al (2018) published the first well‐resolved phylogeny of diploid European members of the Chamaetia / Vetrix clade. Followed by an enlarged study on the origin of polyploidy within Eurasian shrub willows (Wagner et al, 2020) and a study on hexaploid shrub willows of the European Alps (Wagner et al, 2023). RAD‐sequencing data was also used to analyze the species radiation in the Hengduan Mountain System (He et al, 2021).…”

Understanding Arctic–Alpine Plants from Ecological and Evolutionary Perspectives

“…Polyploidy is an important phenomenon in understanding the biogeography and taxonomy of disjunct arctic–alpine species complexes. Wagner et al [ 8 ] provided detailed insights into taxonomically challenging hexaploid alpine shrub willows ( Salix sp., Salicaceae ). In this study, RAD sequencing data, infrared spectroscopy, and morphometric data were used to analyze the phylogenetic relationships of the species from the Phylicifoliae and Nigricantes sections in a framework of 45 Eurasian Salix species.…”