Phylogenetic Relationships of American Willows (<i>Salix</i> L., Salicaceae)

Salix L. is the largest genus in the family Salicaceae (450 species). Several classifications have been published, but taxonomic subdivision has been under continuous revision. Our goal is to establish the phylogenetic structure of the genus using molecular data on all American willows, using three DNA markers. This complete phylogeny of American willows allows us to propose a biogeographic framework for the evolution of the genus. Material was obtained for the 122 native and introduced willow species of America. Sequences were obtained from the ITS (ribosomal nuclear DNA) and two plastid regions, matK and rbcL. Phylogenetic analyses (parsimony, maximum likelihood, Bayesian inference) were performed on the data. Geographic distribution was mapped onto the tree. The species tree provides strong support for a division of the genus into two subgenera, Salix and Vetrix. Subgenus Salix comprises temperate species from the Americas and Asia, and their disjunction may result from Tertiary events. Subgenus Vetrix is composed of boreo-arctic species of the Northern Hemisphere and their radiation may coincide with the Quaternary glaciations. Sixteen species have ambiguous positions; genetic diversity is lower in subg. Vetrix. A molecular phylogeny of all species of American willows has been inferred. It needs to be tested and further resolved using other molecular data. Nonetheless, the genus clearly has two clades that have distinct biogeographic patterns.柳属（Salix L.）是杨柳科（Salicaceae）最大的属，总计包含450个物种。尽管已有多种分类系统被提出，但该属的分类学划分始终处于持续修订之中。本研究以美洲所有柳属物种为研究对象，借助3种DNA分子标记构建该属的系统发育结构。此项针对美洲柳属的完整系统发育研究，可为该属的演化搭建生物地理学研究框架。研究共采集了美洲地区122个本土及外来引种柳属物种的样本，序列扩增自内转录间隔区（ITS，核糖体核DNA）以及matK与rbcL两个质体区域。研究者对所得数据开展了简约法、最大似然法以及贝叶斯推断三种系统发育分析，并将各物种的地理分布信息投射至构建所得的系统发育树之上。该物种树为柳属划分为两个亚属——Salix亚属与Vetrix亚属——提供了强有力的支持依据。Salix亚属包含分布于美洲与亚洲的温带物种，其间断分布格局可能形成于第三纪地质事件。Vetrix亚属则由北半球的寒带-北极分布物种构成，其物种辐射演化可能与第四纪冰期活动相契合。有16个物种的系统发育位置尚不明确；Vetrix亚属的遗传多样性水平更低。本研究已完成美洲所有柳属物种的分子系统发育推断，后续需通过其他分子数据对该结果进行验证与进一步解析。尽管如此，柳属明确存在两个具有截然不同生物地理分布模式的演化支。