Postglacial colonization and parallel evolution of metal tolerance in the polyploid Cerastium alpinum

The Fennoscandian flora is characterized by a high frequency of polyploids, probably because they were more successful than diploid plants in colonizing after the last Ice Age. The first postglacial colonizers were likely poor competitors and became displaced from the lowlands as forests advanced. Consequently, many of these pioneers are currently found only above tree line. However, some have persisted within the forests on open habitats such as naturally toxic serpentine soils where succession is arrested at the pioneer stage. These populations represent relicts of former widely distributed plants. The polyploid Cerastium alpinum L. (Caryophyllaceae) grows on serpentine soils throughout Fennoscandia. C. alpinum populations on different soil types provide a model system for the study of the early postglacial colonization history of Fennoscandia. Genetic markers showed that C. alpinum populations in western Fennoscandia differ genetically from eastern populations, suggesting a two-way colonization. The two lineages meet in a hybrid zone in Northern Scandinavia where a high degree of genetic variation was found. Plants from Fennoscandia and the Western Arctic (Canada, Greenland and Iceland) shared many AFLP fragments, which suggests they originate from common refugia. The Fennoscandian populations were more distantly related to the populations in potential refugia in southern Europe. In fact, the northern populations contained AFLP fragments not found in populations in the Pyrenees and the Alps. Lack of chloroplast DNA variation indicates fast postglacial range expansions and/or a recent origin of C. alpinum. Crosses were made to establish the inheritance of enzyme markers. The results strengthen the evidence for an allopolyploid origin of C. alpinum. Adjacent serpentine and non-serpentine populations of C. alpinum provide a model system of natural replicates to test whether adaptation to serpentine is constitutive (common for all populations) or locally evolved. A growth experiment with high concentrations of nickel and magnesium, two metals that limit the fertility of serpentine soils, showed that the degree of metal tolerance reflects site-specific soil conditions. Since local adaptation was found in both the eastern and the western immigration lineages, the postglacial colonization of Fennoscandia has involved parallel evolution of metal tolerance in C. alpinum.