NATIVE OAK CHLOROPLASTS REVEAL AN ANCIENT DIVIDE ACROSS EUROPE

Glacial refugia and postglacial migration are major factors responsibl e for the present patterns of genetic variation we see in natural popu lations. Traditionally postglacial history has been inferred from foss il data, but new molecular techniques permit historical information to be gleaned from present populations. The chloroplast tRNA(Leu1) intro n contains regions which have been highly conserved over a billion yea rs of chloroplast evolution. Surprisingly, in one of these regions whi ch has remained invariant for all photosynthetic organisms so far stud ied, we have found intraspecific site polymorphism. This polymorphism occurs in two European oaks, Quercus robur and Q. petraea, indicating hybridisation and introgression between them. Two distinct chloroplast types occur and are distributed geographically as eastern and western forms suggesting that these oaks are each derived from at least two s eparate glacial refugia.