title: Diversity and endemism in deglaciated areas: ploidy, relative genome size and niche differentiation in the Galium pusillum complex (Rubiaceae) in Northern and Central Europe.
abstract: OBJECTIVE
  Plants endemic to areas covered by ice sheets during the last glaciation represent paradigmatic examples of rapid speciation in changing environments, yet very few systems outside the harsh arctic zone have been comprehensively investigated so far. The Galium pusillum aggregate (Rubiaceae) is a challenging species complex that exhibits a marked differentiation in boreal parts of Northern Europe. As a first step towards understanding its evolutionary history in deglaciated regions, this study assesses cytological variation and ecological preferences of the northern endemics and compares the results with corresponding data for species occurring in neighbouring unglaciated parts of Central and Western Europe.
  
  
  METHODS
  DNA flow cytometry was used together with confirmatory chromosome counts to determine ploidy levels and relative genome sizes in 1158 individuals from 181 populations. A formalized analysis of habitat preferences was applied to explore niche differentiation among species and ploidy levels.
  
  
  RESULTS
  The G. pusillum complex evolved at diploid and tetraploid levels in Northern Europe, in contrast to the high-polyploid evolution of most other northern endemics. A high level of eco-geographic segregation was observed between different species (particularly along gradients of soil pH and competition) which is unusual for plants in deglaciated areas and most probably contributes to maintaining species integrity. Relative monoploid DNA contents of the species from previously glaciated regions were significantly lower than those of their counterparts from mostly unglaciated Central Europe, suggesting independent evolutionary histories.
  
  
  CONCLUSIONS
  The aggregate of G. pusillum in Northern Europe represents an exceptional case with a geographically vicariant and ecologically distinct diploid/tetraploid species endemic to formerly glaciated areas. The high level of interspecific differentiation substantially widens our perception of the evolutionary dynamics and speciation rates in the dramatically changing environments of Northern Europe.
authors.affiliation: Department of Botany, Faculty of Science, Charles University in Prague, Benátská 2, Prague, Czech Republic.
authors.forename: Filip
authors.initials: F
authors.lastname: Kolár
authors.forename: Magdalena
authors.initials: M
authors.lastname: Lucanová
authors.forename: Petr
authors.initials: P
authors.lastname: Vít
authors.forename: Tomás
authors.initials: T
authors.lastname: Urfus
authors.forename: Jindrich
authors.initials: J
authors.lastname: Chrtek
authors.forename: Tomás
authors.initials: T
authors.lastname: Fér
authors.forename: Friedrich
authors.initials: F
authors.lastname: Ehrendorfer
authors.forename: Jan
authors.initials: J
authors.lastname: Suda
mesh_terms: D017753:Ecosystem; D005060:Europe; D051456:Gene Flow; D014644:Genetic Variation; D005843:Geography; D011003:Ploidies; D019666:Rubiaceae / Q000235:genetics*
keywords: Endemism; environmental change; flow cytometry; genome size; glaciation; niche differentiation; ploidy level; polyploid evolution