Abstract
The genus Salix (willows), with 33 species, represents the most diverse genus of woody plants in the European Alps. Many species dominate subalpine and alpine types of vegetation. Despite a long history of research on willows, the evolutionary and ecological factors for this species richness are poorly known. Here we will review recent progress in research on phylogenetic relationships, evolution, ecology, and speciation in alpine willows. Phylogenomic reconstructions suggest multiple colonization of the Alps, probably from the late Miocene onward, and reject hypotheses of a single radiation. Relatives occur in the Arctic and in temperate Eurasia. Most species are widespread in the European mountain systems or in the European lowlands. Within the Alps, species differ ecologically according to different elevational zones and habitat preferences. Homoploid hybridization is a frequent process in willows and happens mostly after climatic fluctuations and secondary contact. Breakdown of the ecological crossing barriers of species is followed by introgressive hybridization. Polyploidy is an important speciation mechanism, as 40% of species are polyploid, including the four endemic species of the Alps. Phylogenomic data suggest an allopolyploid origin for all taxa analyzed so far. Further studies are needed to specifically analyze biogeographical history, character evolution, and genome evolution of polyploids.
Highlights
Salix L. is the biggest genus in the family Salicaceae, comprising about 450 tree and shrub species that are mostly distributed in the Northern hemisphere
The East–West pattern in the Alps of S. waldsteiniana/S. foetida and S. alpina/S. breviserrata were traditionally recognized as vicariant pairs [11], but it is unclear whether an actual geographical or climatic barrier, e.g., the ice-shields during glaciations of the Pleistocene, separated these species or whether they originated in mountain areas outside the Alps and reached the Alps via long-distance dispersal (LDD)
Three of them (S. mielichhoferi, S. hegetschweileri, and S. glaucosericea) exhibit high ploidy levels and according to Restriction site Associated DNA (RAD) sequencing analyses probably have a complex evolutionary allopolyploid origin, potentially involving more than two parental species [7]. Due to their hybrid origin, the relationships of these species within the Chamaetia/Vetrix clade cannot yet be pinpointed, but there is no obvious relationship of these high polyploids to other lineages of the European Alpine System
Summary
Salix L. is the biggest genus in the family Salicaceae, comprising about 450 tree and shrub species that are mostly distributed in the Northern hemisphere. Many species inhabit high mountain systems, and the genus is highly diverse in the European Alps with approximately 33 species (Table 1; [1]). Four alpine willows are endemic to the Alps, while most species have disjunct distributions in other European mountain systems and in the Arctic [1,5]. Interspecific hybridization might lead to the evolution of new species if robust reproductive barriers are present [17] Such opportunities arise frequently in secondary contact hybrid zones, when previously isolated and diverged species come into contact again. In the Alps, the many range fluctuations of species during the climatic oscillations of the Pleistocene have provided multiple secondary contact zones, as plants were forced to retreat into refugial areas during glacial periods, while warmer periods allowed recolonization of the alpine chain [19].
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