Where are the glacial refugia in Europe? Evidence from pteridophytes

Abstract

In this paper we demonstrate that, by investigating polyploid complexes inAsplenium, it is possible to locate the areas in Europe that are southern glacial refugia, and are likely to have been so since the beginning of the Pleistocene during the consecutive cold and warm periods in Europe. Identification and conservation of these specific areas that serve as safe havens for plants, and perhaps animals, is of paramount importance for the maintenance of European biodiversity because Man's activities are resulting in an ever-increasing loss of natural habitats and putting diversity at risk. The genusAspleniumin Europe comprises some 50 taxa: half of these are diploid while the other half are polyploids derived from the diploids. All aspleniums in Europe are (small) rock ferns with high substrate specificity. Today, most of mainland Europe, Scandinavia and the British Isles has been colonized by polyploidAspleniumspecies, while the diploids that gave rise to these polyploids are distributed around (and more or less confined to) the Mediterranean Basin. In the tetraploids genetic variation is partitioned mostly between sites, whereas diploids show a high degree of genetic variation both within and between sites. The tetraploid taxa seem capable of single spore colonization via intragametophytic selfing, but the diploid taxa appear to be predominantly outbreeding. For most diploids at least two gametophytes, produced by different spores, have to be present to achieve fertilization and subsequent sporophyte formation for the successful colonization of a new site. This results in a slower rate of colonization. The formation of auto- and allopolyploid taxa from diploid communities appears to have been a recurrent and common feature in Europe. Minority cytotype exclusion is likely to prevent the establishment of tetraploids within the diploid communities, but spores from tetraploids can establish populations outside the diploid communities. The differences between colonization abilities of tetraploid and ancestral diploid taxa, resulting from their different breeding systems, has prevented the merging and mingling of their ranges and led to the establishment of contact/hybrid zones. This has resulted in the restriction of diploid populations to ancient glacial refugia and the colonization of the rest of Europe by polyploids. Mapping the current distribution of these diploid communities and comparing the genetic diversity within and between outbreeding diploidAspleniumtaxa allows us to define the area, age and historical biogeography of these refugia and to assess their importance for present day genetic and species diversity in Europe.