Phylogenetic Relationships of American Willows (Salix L., Salicaceae)

Abstract

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.

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“…However, the same lack of resolution was observed with our ITS2 data and in a more detailed analysis of 22 species [26], indicating that the lack of divergence extends into the nuclear genome. This difficulty in differentiating Salix species using molecular markers may reflect hybridization, introgression, recent speciation, allele surfing via range expansion, and low rates of molecular evolution [52; 59; 67; 68; 69]. More extensive phylogenetic studies targeting nuclear markers or whole plastid genomes are necessary to clarify the processes driving the unusually low divergences in Salix .…”

RAD sequencing resolved phylogenetic relationships in European shrub willows (Salix L. subg. Chamaetia and subg. Vetrix) and revealed multiple evolution of dwarf shrubs

“…However, hybridization is an extensively reported phenomenon in Salix and occurs even between distantly related species of different subgenera (Argus, ; Hörandl et al., ; Skvortsov, ). The Vetrix‐Chamaetia clade started to diversify in the late Oligocene (23.7 mya; Wu et al., ), but more comprehensive sequencing data suggest that its major radiation may coincide with Quaternary radiations (Lauron‐Moreau et al., ). Hence, we regard ancient reticulate evolution and conflicting phylogenetic signals a plausible hypothesis for the observed unresolved tree topologies.…”

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