Genetic structure of newly established populations of Cryphonectria parasitica

The genetic structure of three newly established Cryphonectria parasitica p opulations (Choex, Weggis, Murg) was analysed and compared to an older, pos t-epidemic population (Claro). Vegetative compatibility (vc) type and DNA f ingerprint analysis revealed an almost clonal C. parasitica population in C hoex, Weggis and Murg. These stands are all situated in northern Switzerlan d, outside the main range of European chestnut. Only one ve type and one do minant DNA fingerprint was found in Choex and Murg. In Weggis, two ve types and two dominating DNA fingerprints were found. The European Cryphonectria hypovirus (CHV1) was not detected in these three populations. In contrast, the population in Claro, situated within the main range of European chestn ut in southern Switzerland, had much higher vc type and DNA fingerprint div ersity. DNA fingerprints were correlated to vc types in Claro and in Weggis . Mating type determination revealed one strongly dominating mating type in each of the three northern populations, but not in Claro. From these resul ts we conclude that C, parasitica disseminated almost exclusively by means of asexual reproduction in Choex, Weggis and Murg, whereas in Claro sexual reproduction also played an important role. Additionally, founder effects a nd restricted gene flow were dominating factors in shaping the genetic stru cture of the three northern populations.