Reticulate evolution and hybridization in willows (Salix L.)

杂交是开花植物的一个重要进化过程，可能导致新物种的形成。杂交物种形成需要建立一种针对双亲的生殖屏障，例如通过占据新的生态位或通过基因组变化。如果亲本物种的亲缘关系较远，杂交物种形成的可能性似乎更大。然而，成功的杂交物种形成的案例很少有文献记载；关于古代杂交的现有知识和成功建立杂交谱系的情况仍然知之甚少。在这里，我们建议对模式系统柳树(柳树)进行基因组研究，该系统长期以来一直被认为具有极高的种间杂交频率。来自不同分类组的远亲物种之间的杂交在自然种群中定期形成。关于系统发育关系、形态模式和多倍体存在的信息表明，该属的进化史是由网状进化和古代杂交物种形成形成的。在现存物种的杂交种中，一些远缘杂交似乎在冰川前野的极端栖息地建立了新的、独立的种群。在这个项目中，我们将使用限制相关DNA测序(RADseq)作为一种强大的下一代测序方法来解决浅层和复杂的系统发育问题。通过从全基因组中挖掘大量高质量的单核苷酸多态(SNPs)，我们将重建物种的系统发育关系、杂合性、杂交性和亲缘关系。对系统发育的测年将使人们对物种形成事件的分化时间和年龄有更深入的了解。该项目的目标是：i)重建灌木柳树在来自欧洲18个组的代表性物种上的系统发育关系和进化史，以检验柳树古代杂交物种形成的假说；ii)以最近在阿尔卑斯山建立的二级接触杂交区为例，我们将通过扩大抽样研究杂交种的遗传结构、世代结构、回交和导入，以检验这样的种群是柳树杂交进化的模式系统的假说。

Hybridization is an important evolutionary process in flowering plants and potentially can result in the formation of new species. Hybrid speciation requires the establishment of a reproductive barrier against the parents, e.g., via occupation of novel ecological niches or via genomic changes. Hybrid speciation appears to be more likely if parental species were distantly related. However, few cases of successful hybrid speciation are documented; the present knowledge on ancient hybridization and circumstances of successful establishment of hybrid lineages are still poorly understood. Here we propose a genomic study on the model system Salix (willows), which has long been known to exhibit exceptional high frequencies of interspecific hybridization. Crosses between distantly related species from different taxonomic sections form regularly in natural populations. Information on phylogenetic relationships, morphological patterns, and presence of polyploidy suggest that the evolutionary history of the genus has been shaped by reticulate evolution and ancient hybrid speciation. Among hybrids of extant species, some wide crosses appear to establish novel, self-standing populations on extreme habitats of glacier fore fields. In this project we will use restriction-associated DNA sequencing (RADseq) as a powerful next-generation sequencing method for resolving shallow and complex phylogenies. By mining of a large number of high-quality single-nucleotide polymorphisms (SNPs) out of the whole genome we will reconstruct phylogenetic relationships, heterozygosity, hybridity and parentage of species. Dating of the phylogeny will give insights into divergence time and age of speciation events. The project goals are: i) to reconstruct phylogenetic relationships and evolutionary history of shrub willows on representative species from 18 European sections to test the hypothesis of ancient hybrid speciation in willows; ii) on a case studies of a recently established secondary contact hybrid zone in the Alps, we will study on an expanded sampling the genetic structure, generation structure, backcrossing and introgression of hybrids to test the hypothesis that such populations represent a model system for hybrid evolution in willows.