CHLOROPLAST DNA FOOTPRINTS OF POSTGLACIAL RECOLONIZATION BY OAKS

Recolonization of Europe by forest tree species after the last glaciat ion is well documented in the fossil pollen record. This spread may ha ve been achieved at low densities by rare events of long-distance disp ersal, rather than by a compact wave of advance, generating a patchy g enetic structure through founder effects, In long-lived oak species, t his structure could still be discernible by using maternally transmitt ed genetic markers, To test this hypothesis, a fine-scale study of chl oroplast DNA (cpDNA) variability of two sympatric oak species was carr ied out in western Prance, The distributions of six cpDNA length varia nts were analyzed at 188 localities over a 200 x 300 km area, A cpDNA map was obtained by applying geostatistics methods to the complete dat a set. Patches of several hundred square kilometers exist which are vi rtually fixed for a single haplotype for both oak species. This local systematic interspecific sharing of the maternal genome strongly sugge sts that long-distance seed dispersal events followed by interspecific exchanges were involved at the time of colonization, about 10,000 yea rs ago.