RAD sequencing data reveal a radiation of willow species (SalixL., Salicaceae) in the Hengduan Mountains and adjacent areas

Abstract

<jats:title>Abstract</jats:title><jats:p>The Hengduan Mountains (HDM) in South West China are an important hotspot of plant diversity and endemism and considered to be a secondary diversification center for the woody plant genus <jats:italic>Salix</jats:italic> (Salicaceae). This study aimed to reconstruct the spatio-temporal evolution of the <jats:italic>Salix Chamaetia</jats:italic>-<jats:italic>Vetrix</jats:italic> clade in the HDM and to test for the occurrence of a radiation. We inferred phylogenetic relationships based on more than 34,000 RAD loci of 27 species. Phylogenetic analyses recovered a well-resolved tree topology with two major clades, the Eurasian and the HDM clade and a divergence time of c. 23.9 Ma. The HDM clade comprises two subclades. The species of the HDM clade originated in north HDM and adjacent areas and then dispersed into the south HDM, westwards to the Himalayas and eastwards to the Qinling Mountains. Niche modelling analyses revealed that during the last glacial maximum, range contractions were observed in the northern areas, while southward expansions resulted in range overlaps. The reconstruction of putative adaptive character evolution of plant height, inflorescence and flower morphology indicate that adaptations to altitudinal distribution contributed to the diversification of the HDM willows. Our data indicate that a radiation occurred in HDM within the <jats:italic>Salix Chamaetia</jats:italic>-<jats:italic>Vetrix</jats:italic> clade. Dispersal within the mountain system and to adjacent regions as well as survival in glacial refugia have shaped the biogeographical history of the clade. Differentiation along altitudinal zonation concomitant to morphological adaptations to colder climates may be important ecological factors for the high species diversity of <jats:italic>Salix</jats:italic> in this area.</jats:p>

Phylogenomic Relationships and Evolution of Polyploid Salix Species Revealed by RAD Sequencing Data

“…Our findings support the suitability of RAD sequencing for interspecific phylogenetic inference and are in accordance with many studies on different levels of divergence (e.g. Hipp et al., 2014 ; Eaton et al., 2017 ; Wagner et al., 2018 ; He et al., 2020 ). The allelic information of the polyploid samples can be condensed to a single consensus sequence to circumvent the challenges of dealing with more than two alleles for phylogenetic approaches.…”

“…The ploidy levels of seven species (23 samples) with unknown chromosome counts were estimated using flow cytometric analyses based on DAPI (4’, 6-diamidino-2-phenylindole) fluorochrome applying a modified protocol of Suda and Trávníček (2006) as described in He et al. (2020) .…”

“…The huge amount of biallelic SNPs is well suited to study evolutionary processes, resolve reticulate relationships, reconstruct species trees, and to analyze polyploid species in a phylogenetic framework ( Bryant et al., 2012 ; Dufresne et al., 2014 ; Clevenger et al., 2015 ; Clevenger et al., 2018 ). However, there are only few studies using RAD sequencing data when dealing with polyploid non-model plants ( Chen et al., 2013 ; Mastretta-Yanes et al., 2014 ; Qi et al., 2015 ; Brandrud et al., 2020 ; He et al., 2020 ).…”

“…RAD sequencing ( Baird et al., 2008 ) was recently used to overcome the lack of phylogenetic information within the genus Salix ( Gramlich et al., 2018 ; Wagner et al., 2018 ; He et al., 2020 ). Wagner et al.…”

“… Wagner et al. (2018) published the first well-resolved phylogeny of diploid European members of the Chamaetia / Vetrix clade, while He et al. (2020) recognized a recent adaptive radiation of willows in the Hengduan mountains.…”