Phylogenetic relationships and evolution of high mountain buttercups (Ranunculus) in North America and Central Asia

The evolutionary history of high mountain plants in the Northern hemisphere has frequently been shaped by geographical isolation and allopatric speciation. In sympatry, high mountain systems potentially provide opportunities for ecological speciation along altitudinal belts and via polyploidy. However, the role of habitat and cytotype differentiation as isolation mechanisms is poorly understood. We developed a well-resolved phylogenetic framework for a clade of 34 North American and Central Asian mountain Ranunculi to reconstruct sister relationships and ecogeographical patterns. Based on seven DNA sequence markers (ITS of nrDNA, rpl32-trnL, trnT-trnL, ndhA intron, petL-psbE, trnV-ndhC, matK/trnK, and the psbJ-petA intergenic spacer) we utilized Maximum Likelihood and NeighborNet analyses. Data on habitat, altitude and ploidy levels were mapped onto the phylogenetic tree by using Mesquite analysis, and ecological differentiation was tested by habitat contrasts of sister species and by pairwise matching analysis. Phylogenetic analysis revealed five well supported geographical subclades (two in North America, each one in Southern Central Asia, northern Central Asia/Arctic and in European/Asian lowland). The North American sister species exhibit allopatric, parapatric and sympatric distributions and represent even in sympatry distinct genetic clusters with some degree of habitat and/or cytotype differentiation. In the Himalayas and in the mountains of Taiwan all sister taxa occur in sympatry and form multiple genetic clusters; habitat differentiation is significantly lower than in North America, while cytotype diversity is poorly documented. In the temperate to arctic mountains of North America, ecological differentiation along altitudinal belts and/or cytotype differentiation may contribute to maintenance of species boundaries in sympatry. In the Himalayas, the restriction of buttercups to higher altitudes probably hampers ecological differentiation; hence, ongoing hybridization blurs species boundaries in sympatry. Isolation mechanisms in high mountain systems cannot be generalized but depend on the peculiar geomorphological and climatic conditions.