Single sequence repeat markers reflect diversity and geographic barriers in Eurasian populations of the conifer pathogen Ceratocystis polonica

The blue‐stain fungus and vascular stain pathogen Ceratocystis polonica and its associated bark beetle vectors, particularly Ips typographus and I. typographus japonicus, cause significant losses to several spruce species in Eurasia. Nothing is, however, known about the population genetics of this conifer pathogen. In this study, a set of single sequence repeat (SSR) markers were developed to determine the population structure and genetic diversity of C. polonica in Europe and Japan. ISSR‐PCR primers were used to target SSR‐rich regions and specific primers were designed flanking the SSR regions found in these amplicons. The SSR primers developed for C. polonica were found to be transferable to six other Ceratocystis species from conifers, residing in the Ceratocystis coerulescens complex. Ninety‐eight isolates representing four populations of C. polonica (Austria, Norway, Poland and Japan) were tested using 10 selected polymorphic SSR markers. A high level of gene diversity was found in C. polonica as a whole (H = 0.53). Analysis of G statistics showed a low degree of population structure in Europe and a high level of gene flow between populations (Gst = 0.05, Nm = 8.5). In contrast, the Japanese and the European populations of C. polonica displayed strong genetic separation, which is likely caused by geographic isolation. The low level of population structure of C. polonica in Europe and the differentiation between the European and the Japanese fungal populations mirror previous findings for I. typographus and I. typographus japonicus, the main insect vectors of this fungus. These results support the view that the fungus and the insect have closely co‐evolved together. This study also suggests that movement of C. polonica and its vectors between Europe and Asia pose a threat to forestry on both continents and this should clearly be avoided.