Spatio-temporal evolution of shrub willows (Salix subgenus Vetrix clade) on a global scale

Le résumé fourni par la source

• Salix species of the Vetrix clade group into four lineages that show a geographical pattern: The Pan-Himalayan clade, the North American clade, and two Eurasian clades. • Conflicting signals in the backbone and fine-scale relationships highlight the presence of reticulation in the Vetrix clade. • Holarctic shrub willow species evolved in the Tertiary and diversified with global cooling at the Eocene–Oligocene boundary. • Two species radiations in the Pan-Himalayan region and in North America contributed to the observed species diversity. • Migration events mainly occurred within the Old World and the New World, respectively. • Arctic lineages are older than the habitats they populate suggesting preadaptations and subsequent dispersal to the emerging arctic regions. The spatio-temporal evolution of woody plant lineages in the Holarctic is still understudied, limiting our understanding of evolutionary processes that promote higher diversity at higher latitudes in some lineages, which contradicts the latitudinal gradient hypothesis which predicts higher species richness in the tropics. Shrub willows ( Salix subgenus Vetrix ) comprise about 350 species of which many are adapted to arctic-alpine ecosystems and thus form an important element across the Holarctic region. In this study, we utilized RAD sequencing to estimate the spatio-temporal evolution of the Vetrix clade based on about 140 species covering most of its morphological and geographical diversity. The resolved phylogeny revealed four lineages: the Pan-Himalayan clade, the North American clade and two clades containing species from Eurasia. Widespread circumpolar species form a hybridogenetic grade between the Eurasian and North American clades. Our results confirm that shrub willows originated in the Tertiary, probably in Asia, and that diversification coincided with the climatic cooling in the Pleistocene. At least two radiations were observed, in the Pan Himalayas and in North America. Speciation was further shaped by migration and dispersal in Eurasia, likely accelerated by the uplift of mountain chains, the closure of the Turgai Strait and expansion of suitable habitats. The dated phylogeny revealed that speciation of arctic lineages predates adaptation to high latitudes, as also evident from the fossil record. Small wind-dispersed seeds enabled shrub willows to colonize the expanding arctic tundra regions in the Miocene and their recolonization after the LGM. Given the high observed number of polyploid species equally distributed in all clades and indications of genetic admixture, we assume that species diversity in shrub willows was additionally increased by hybridization and polyploidization.

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