Cryptic Speciation in Western North America and Eastern Eurasia of the Pathogens Responsible for Laminated Root Rot

Coniferiporia sulphurascens is a facultative fungal pathogen that causes laminated root rot (LRR) in commercially important coniferous species worldwide. This fungus spreads primarily by way of vegetative mycelium transferring at points of contact between infected and healthy roots. Successful intervention to control LRR requires a better understanding of the population structure and genetic variability of C. sulphurascens. In this study, we investigated the population genetic structure and origin of C. sulphurascens populations in western North America and eastern Eurasia collected from multiple coniferous hosts. By analyzing the small and large mitochondrial ribosomal RNA subunit genes combined with six nuclear loci (internal transcribed spacer region, actin, RNA polymerase II largest subunit, RNA polymerase II second-largest subunit, laccase-like multicopper oxidase, and translation elongation factor 1-α), we observed that none of the alleles among the loci were shared between North American (NA) and Eurasian C. sulphurascens populations. In total, 55 multilocus genotypes (MLGs) were retrieved in C. sulphurascens isolates occurring in these two continental regions. Of these, 41 MLGs were observed among 58 isolates collected from widespread locations in British Columbia (Canada) and the northwestern United States, while 14 MLGs were observed among 16 isolates sampled in Siberia and Japan. Our data showed that the levels of genetic differentiation between the NA and Eurasian populations are much greater than the populations from within each continental region; the two continental populations formed clearly divergent phylogenetic clades or lineages since they were separated approximately 7.5 million years ago. Moreover, the Eurasian population could be the source of the NA population. Our study indicates the existence of cryptic diversity in this pathogen species, and strongly suggests that the NA and Eurasian populations represent two lineages, which have progressively diverged from each other in allopatry.