Reticulate evolution and hybridization in willows (Salix L.)

杂交是显花植物重要的进化过程，并可能导致新物种的形成。杂交物种形成需要建立一个对父母的生殖障碍，例如，通过占据新的生态位或通过基因组变化。杂交物种形成似乎更有可能，如果父母的物种是远亲关系。然而，很少有成功的杂交物种形成的记录，目前的知识古代杂交和成功建立杂交谱系的情况仍然知之甚少。在这里，我们提出了一个基因组研究的模式系统柳（柳树），这一直是众所周知的表现出异常高频率的种间杂交。来自不同分类学部分的远亲物种之间的杂交在自然种群中经常形成。系统发育关系，形态模式和多倍体的存在的信息表明，该属的进化历史已经形成的网状进化和古老的杂交物种。在现存物种的杂交种中，一些广泛的杂交似乎建立了新的，独立的种群在冰川前场的极端栖息地。在这个项目中，我们将使用限制性相关DNA测序（RADseq）作为解决浅层和复杂基因组的强大的下一代测序方法。通过从整个基因组中挖掘大量高质量的单核苷酸多态性（SNPs），我们可以重建物种的系统发育关系、杂合性、杂交性和亲子关系。定年的物种形成将有助于了解物种分化的时间和年龄。该项目的目标是：（1）以欧洲18个代表种为样本，重建了灌木柳树的系统发育关系和进化历史，以验证柳树古老杂交物种形成的假说; ii）在阿尔卑斯山最近建立的次级接触杂交区的案例研究中，我们将在扩大取样的基础上研究遗传结构、世代结构，杂种的回交和渐渗以检验这样的群体代表柳树中杂种进化的模型系统的假设。

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.