Abstract
The time frame and geographical patterns of diversification processes in European temperate-montane herbs are still not well understood. We used the sexual species of the Ranunculus auricomus complex as a model system to understand how vicariance versus dispersal processes in the context of Pleistocene climatic fluctuations have triggered speciation in temperate-montane plant species. We used target enrichment sequence data from about 600 nuclear genes and coalescent-based species tree inference methods to resolve phylogenetic relationships among the sexual taxa of the complex. We estimated absolute divergence times and, using ancestral range reconstruction, we tested if speciation was enhanced by vicariance or by dispersal processes. Phylogenetic relationships among taxa were fully resolved with some incongruence in the position of the tetraploid R.marsicus. Speciation events took place in a very short time at the end of the Mid-Pleistocene Transition (830-580 thousand years ago [ka]). A second wave of intraspecific geographical differentiation occurred at the end of the Riss glaciation or during the Eemian interglacial between 200 and 100ka. Ancestral range reconstruction suggests a widespread European ancestor of the R.auricomus complex. Vicariance has triggered allopatric speciation in temperate-montane plant species during the climatic deterioration that occurred during the last phase of the Mid-Pleistocene Transition. Vegetation restructuring from forest into tundra could have confined these forest species into isolated glacial macro- and microrefugia. During subsequent warming periods, range expansions of these species could have been hampered by apomictic derivatives and by other congeneric competitors in the same habitat.
Highlights
Pleistocene climatic fluctuations have caused periodical range shifts and/or range expansions and contractions in northern hemisphere plant species
Range expansions and the consequent formation of secondary contact zones resulted in hybridization, and eventually in the formation of new taxa through allopolyploidization (Stebbins, 1984; Abbott et al, 2013)
The average percent of mapped reads was 70.14%, with mapping success ranging from 53% to 82%, and only one sample (R. notabilis s.l.; 2) exhibiting a considerably lower number of mapped reads (26.89%; Table S4)
Summary
Pleistocene climatic fluctuations have caused periodical range shifts (e.g., north-south) and/or range expansions and contractions in northern hemisphere plant species. In Europe, cold periods have produced southward shifts in the distribution ranges of temperate species (in the so-called “Mediterranean refugia”; Taberlet, Fumagalli, Wust-Saucy, & Cosson, 1998; Brewer et al, 2002) and/or survival of these in few, isolated central European refugial areas (e.g., Magri et al, 2006; Naydenov, Senneville, Beaulieu, Tremblay, & Bousquet, 2007). Warmer interglacials have favoured range expansion towards north and the formation of contact zones among diverged populations of the same species (Hewitt, 1999; Magri, 2008). The survival of formerly coherent population groups in isolated refugia promoted allopatric speciation. Range expansions and the consequent formation of secondary contact zones resulted in hybridization, and eventually in the formation of new taxa through allopolyploidization (Stebbins, 1984; Abbott et al, 2013)
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