The origin of unique diversity in deglaciated areas: traces of Pleistocene processes in north‐European endemics from the Galium pusillum polyploid complex (Rubiaceae)

Abstract

The role of glacial oscillations in shaping plant diversity has been only rarely addressed in endemics of formerly glaciated areas. The Galium pusillum group represents a rare example of an ecologically diverse and ploidy-variable species complex that exhibits substantial diversity in deglaciated northern Europe. Using AFLP and plastid and nuclear DNA sequences of 67 populations from northern, central, and western Europe with known ecological preferences, we elucidate the evolutionary history of lineages restricted to deglaciated areas and identify the eco-geographic partitioning of their genetic variation. We reveal three distinct endemic northern lineages: (i) diploids from southern Sweden + the British Isles, (ii) tetraploids from southern Scandinavia and the British Isles that show signs of ancient hybridization between the first lineage and populations from unglaciated central Europe, and (iii) tetraploids from Iceland + central Norway. Available evidence supports a stepwise differentiation of these three lineages that started at least before the last glacial maximum by processes of genome duplication, interlineage hybridization and/or allopatric evolution in distinct periglacial refugia. We reject the hypothesis of more recent postglacial speciation. Ecological characteristics of the populations under study only partly reflect genetic variation and suggest broad niches of postglacial colonizers. Despite their largely allopatric modern distributions, the north-European lineages of the G. pusillum group do not show signs of rapid postglacial divergence, in contrast to most other northern endemics. Our study suggests that plants inhabiting deglaciated areas outside the Arctic may exhibit very different evolutionary histories compared with their more thoroughly investigated high-arctic counterparts.

Ecological niche differences between two polyploid cytotypes of Saxifraga rosacea

“…There are at least three nonexclusive explanations for the complete spatial segregation of the cytotypes of Saxifraga . Minority cytotypes may become quickly excluded in mixed populations (Levin, ), cytotypes may have different ecological requirements (Ramsey, ; Kolář et al., ; McAllister et al., ; Muñoz‐Pajares et al., ), or historical factors such as differences in recolonization history and stochastic events may have shaped the distribution of the cytotypes (Sexton et al., ).…”

“…While some studies have found correlations between cytotype distribution and climate variables and differences in habitat conditions between cytotypes (e.g., Raabová et al., ; Kolář et al., ; Sonnleitner et al., ; Ramsey, ; Richardson and Hanks, ; Mráz et al., ; McAllister et al., ; Visger et al., ; Muñoz‐Pajares et al., ), others have found evidence for shared broad‐scale climatic niches between cytotypes and no evidence for differences in climatic requirements (Godsoe et al., ; Glennon et al., and references therein). Ideally, the study of niche differentiation would include experiments testing the effects of environmental factors on different cytotypes, but there are not many studies involving such experiments (Ramsey, ; Kolář et al., ; McIntyre and Strauss, ). As polyploidization can also drive changes in phenotypic traits in natural populations (Comai, ; Chen and Sun, ), the study of phenotypic differences between different cytotypes may be important to understand their ecology (Segraves et al., ; Nuismer and Cunningham, ; Münzbergova, ).…”

“…A recent review concluded that co-occurrence of different cytotypes within populations of mixed-ploidy species is common, but spatial segregation at different scales can occur (Kolář et al, 2015). In the case of mosaic parapatry, mostly single-cytotype populations may be spatially intermingled, or dominant cytotypes may be spatially separated with limited contact zones (large-scale parapatry; Kolář et al, 2015).…”

“…A recent review concluded that co-occurrence of different cytotypes within populations of mixed-ploidy species is common, but spatial segregation at different scales can occur (Kolář et al, 2015). In the case of mosaic parapatry, mostly single-cytotype populations may be spatially intermingled, or dominant cytotypes may be spatially separated with limited contact zones (large-scale parapatry; Kolář et al, 2015). The pattern of the geographic distribution of cytotypes can provide important insights about the origin and maintenance of different ploidy levels (Baack, 2005;Rieseberg and Willis, 2007;Kolář et al, 2009;Muñoz-Pajares et al, 2018).…”

Adenophora liliifolia: Condition of its Populations in Central Europe

“…(Šmídová et al, 2011) or Pedicularis sudetica Willd. (Hendrych and Hendrychová, 1988), became glacial or postglacial relicts, but also new local endemics appeared, e.g., Galium sudeticum Tausch, G. cracoviense Ehrend., Cochlearia polonica E. Froelich (Cieślak et al, 2007(Cieślak et al, , 2015Szeląg, 2009, 2010;Kolář et al, 2013Kolář et al, , 2015 or Sorbus sudetica (Tausch) Bluff et al (Kaplan, 2012). Climatic changes during the Quaternary Period strongly affected the species composition and the species distribution in Europe (Szafer, 1946(Szafer, -47, 1954Kaplan, 2012).…”