Discovery of a new gene pool and a high genetic diversity of the chestnut blight fungus Cryphonectria parasitica in Caucasian Georgia

- We investigated the population structure of the chestnut blight fungus in Georgia.
- Genotypic diversity is high because of sexual recombination between two main haplotypes.
- One main haplotype most likely spontaneously invaded Georgia from neighboring Turkey.
- The other dominant haplotype has probably directly been introduced from Asia.
- Georgia represents a potential source of diversity for the European Cryphonectriaparasitica population.

In this study, we investigated the population genetic structure and possible origins of the plant pathogen Cryphonectria parasitica in Caucasian Georgia, a region within the centre of origin of the host species Castanea sativa. A total of 427 C. parasitica isolates from nine populations were genotyped at 10 microsatellite loci. A high genetic diversity was detected, but the overall Georgian population was dominated by three haplotypes which were present in most individual populations. Two of them have not been previously found in Europe. Bayesian clustering analysis and principal component analysis could not identify their source population, neither in Asia nor in North America. On the other hand, one haplotype is frequent in Central Europe and probably naturally invaded Caucasian Georgia from neighbouring Turkey. Seventy-three haplotypes were unique to specific populations, and 66 of them were represented by a single isolate. Allele patterns suggest that most of these haplotypes emerged locally through sexual recombination between haplotypes of the Georgian and the central European gene pool. Due to the high incidence of haplotypes not otherwise present in Europe, Caucasian Georgia represents an additional source of diversity for the European C. parasitica population.