Genetically diverse but with surprisingly little geographical structure: the complex history of the widespread herb Carex nigra (Cyperaceae)

Aim Based on extensive range‐wide sampling, we address the phylogeographical history of one of the most widespread and taxonomically complex sedges in Europe, Carex nigra s. lat. We compare the genetic structure of the recently colonized northern areas (front edge) and the long‐standing southern areas (rear edge), and assess the potential genetic basis of suggested taxonomic divisions at the rank of species and below. Location Amphi‐Atlantic, central and northern Europe, circum‐Mediterranean mountain ranges, central Siberia, Himalayas. Methods A total of 469 individuals sampled from 83 populations, covering most of the species’ range, were analysed with amplified fragment length polymorphism (AFLP) and chloroplast DNA (cpDNA) markers. Bayesian clustering, principal coordinates analysis, and estimates of diversity and differentiation were used for the analysis of AFLP data. CpDNA data were analysed with statistical parsimony networks and maximum parsimony and Bayesian inference of phylogenetic trees. Results Overall genetic diversity was high, but differentiation among populations was limited. Major glacial refugia were inferred in the Mediterranean Basin and in western Russia; in addition, there may have been minor refugia in the North Atlantic region. In the southern part of the range, we found high levels, but geographically quite poorly structured genetic diversity, whereas the levels of genetic diversity varied among different areas in the north. North American populations were genetically very similar to the European populations. Main conclusions The data are consistent with extensive gene flow, which has obscured the recent history of the taxon. The limited differentiation in the south probably results from the mixing of lineages expanding from several local refugia. Northward post‐glacial colonization resulted in a leading‐edge pattern of low diversity in the Netherlands, Belgium, Scotland and Iceland, whereas the observed high diversity levels in Fennoscandia suggest broad‐fronted colonization from the south as well as from the east. The patterns found in the American populations are consistent with post‐glacial colonization, possibly even with anthropogenic introduction from Europe. Our data also suggest that the tussock‐forming populations of C. nigra, often referred to as a distinct species (Carex juncella), represent an ecotype that has originated repeatedly from different populations with creeping rhizomes.