Where are the glacial refugia in Europe? Evidence from pteridophytes

In this paper we demonstrate that, by investigating polyploid complexes in Asplenium, it is possible to locate the areas in Europe that are southern g lacial refugia, and are likely to have been so since the beginning of the P leistocene during the consecutive cold and warm periods in Europe. Identifi cation and conservation of these specific areas that serve as safe havens f or plants, and perhaps animals, is of paramount importance for thr maintena nce of European biodiversity because Man's activities are resulting in an e l-cr-increasing loss of natural habitats and putting diversity at risk. The genus Asplenium in Europe comprises some 50 taxa: half of these are diploi d while the other half are polyploids derived from the diploids. All asplen iums in Europe are (small) rock ferns with high substrate specificity. Toda y, most of mainland Europe, Scandinavia and the British Isles has been colo nized by polyploid Asplenium species, 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 mo stly between sites, whereas diploids show a high degree of genetic variatio n both within and between sites. The tetraploid tar;a seem capable of singl e spore colonization via intragametophytic selfing, bur the diploid taxa ap pear to be predominantly outbreeding. For most diploids at It asi two gamet ophytes, produced by different sporophyte have to be present to achieve fer tilization and subsequent sporophyte formation for the successful colonizat ion of a new site. This results in a slower rate of colonization. The forma tion of auto- and allopolyploid taxa from diploid communities appears to ha ve been a recurrent and common feature in Europe. Minority cytotype exclusi on 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 bree ding systems, has prevented the merging and mingling of their ranges and le d to the establishment of contact/hybrid zones. This has resulted in the re striction of diploid populations to ancient glacial refugia and the coloniz ation of the rest of Europe by polyploids. Mapping the current distribution of these diploid communities and comparing the genetic diversity within an d between outbreeding diploid Asplenium taxa allows us in define the area, age and historical biogeography of these refugia and to assess their import ance for present day genetic and species diversity in Europe. (C) 1999 The Linnean Society of London.