CHLOROPLAST DNA RECOGNIZES 3 REFUGIAL SOURCES OF EUROPEAN OAKS AND SUGGESTS INDEPENDENT EASTERN AND WESTERN IMMIGRATIONS TO FINLAND

Refugial differentiation and routes of postglacial migration are major determinants of the patterns of geographical variation we see in natu ral populations today. We used patterns of chloroplast DNA (cpDNA) var iation to investigate the postglacial colonization history of the Euro pean oak species Quercus robur and Q. petraea. By sequencing two cpDNA segments using universal primers, we revealed four polymorphic sites which identify four cytotypes with characteristic geographical distrib utions. Of these, the principal eastern, central and western cytotypes divide the range into three longitudinal zones, each extending from t he south to the north of Europe. This corroborates the idea that the p ostglacial colonization started from three distinct southerly refugia. The fourth cytotype, restricted to East Anglia, was probably derived from the western type postglacially. As a special problem, we addresse d the controversial origin of Q. robur at its northern limits in south -western Finland, where it currently occupies a narrow coastal zone di sjunct from the remaining oak range. Using a PCR-RFLP assay that discr iminates the eastern cytotype, a contact zone of two cytotypes was ide ntified in the region of the Salpausselka ridges. This suggests that t he marginal northern occurrence was independently colonized both from the east and from the west, across the Baltic Sea.