RAD-seq reveals genetic structure of the F2-generation of natural willow hybrids (Salix L.) and a great potential for interspecific introgression

Abstract

BackgroundHybridization of species with porous genomes can eventually lead to introgression via repeated backcrossing. The potential for introgression between species is reflected by the extent of segregation distortion in later generation hybrids. Here we studied a population of hybrids between Salix purpurea and S. helvetica that has emerged within the last 30 years on a glacier forefield in the European Alps due to secondary contact of the parental species. We used 5758 biallelic SNPs produced by RAD sequencing with the aim to ascertain the predominance of backcrosses (F1 hybrid x parent) or F2 hybrids (F1 hybrid x F1 hybrid) among hybrid offspring. Further, the SNPs were used to study segregation distortion in the second hybrid generation.ResultsThe analyses in structure and NewHybrids revealed that the population consisted of parents and F1 hybrids, whereas hybrid offspring consisted mainly of backcrosses to either parental species, but also some F2 hybrids. Although there was a clear genetic differentiation between S. purpurea and S. helvetica (FST = 0.24), there was no significant segregation distortion in the backcrosses or the F2 hybrids. Plant height of the backcrosses resembled the respective parental species, whereas F2 hybrids were more similar to the subalpine S. helvetica.ConclusionsThe co-occurrence of the parental species and the hybrids on the glacier forefield, the high frequency of backcrossing, and the low resistance to gene flow via backcrossing make a scenario of introgression in this young hybrid population highly likely, potentially leading to the transfer of adaptive traits. We further suggest that this willow hybrid population may serve as a model for the evolutionary processes initiated by recent global warming.Electronic supplementary materialThe online version of this article (10.1186/s12870-018-1552-6) contains supplementary material, which is available to authorized users.

Relationships and genome evolution of polyploid Salix species revealed by RAD sequencing data

“…Recent allopolyploidization (young polyploidy) may have happened during the Pleistocene, when climatic oscillations caused range fluctuations of plant species and enhanced secondary contact hybridization of previously isolated species (Hewitt 2004; Abbott et al 2013; Kadereit 2015). While homoploid hybridization may result in introgressive hybridization with the parental species (Hardig et al 2010; Gramlich and Hörandl 2016; Gramlich et al 2018), allopolyploidy can establish a rapid crossing barrier against the parents and hence may result in the evolution of an independent lineage. Cold climatic regimes during the Pleistocene, however, may have also triggered spontaneous autopolyploidization by unreduced gamete formation (Ramsey and Schemske 1998).…”

“…In willows, natural hybridization is a frequent phenomenon (Argus 1997; Skvortsov 1999; Hörandl et al 2012) and may occur even between distantly related species (e.g. Hardig et al 2000; Gramlich et al 2018). However, these processes are not necessarily exclusive.…”

“…This lack of a phylogenetic framework for willows hampered also the reconstruction of relationships of polyploids within the genus. RAD sequencing (Baird et al 2008) was recently used to overcome this lack of information in genus Salix (Gramlich et al 2018; Wagner et al 2018). This technique reveals thousands of genome wide informative SNPs to analyze relationships within and among species (Baird et al 2008; Wagner et al 2018).…”

“…This technique reveals thousands of genome wide informative SNPs to analyze relationships within and among species (Baird et al 2008; Wagner et al 2018). Wagner et al (2018) published the first well-resolved phylogeny of diploid European members of the Chamaetia / Vetrix clade, while Gramlich et al (2018) successfully used RAD sequencing to analyse recent homoploid hybridization patterns and introgression between two Salix species on alpine glacier forefields. RAD sequencing loci in Salix represent almost exclusively the nuclear genome, and provide ten thousands of SNPs from both conservative and rapidly evolving non-coding genomic regions (Gramlich et al 2018).…”

“…Wagner et al (2018) published the first well-resolved phylogeny of diploid European members of the Chamaetia / Vetrix clade, while Gramlich et al (2018) successfully used RAD sequencing to analyse recent homoploid hybridization patterns and introgression between two Salix species on alpine glacier forefields. RAD sequencing loci in Salix represent almost exclusively the nuclear genome, and provide ten thousands of SNPs from both conservative and rapidly evolving non-coding genomic regions (Gramlich et al 2018). Therefore, this method is well suited to resolve reticulate relationships and to analyse the origin of the polyploid species in Salix .…”