Progress and Challenges in Understanding the Biology, Diversity, and Biogeography of Cenococcum geophilum

Cenococcum geophilum (Dothideomycetes, Ascomycota) is one of the most common ectomycorrhizal fungi in boreal and temperate regions. Although C. geophilum was originally considered as a single species, accumulating evidence suggests that C. geophilum is actually a diverse species complex. Here we provide an overview of the current data on global host range, distribution and biogeography of C. geophilum and discuss what is known about the spatial genetic structure at scales from soil cores to biomes to continents. Recent molecular data indicate that the genetic diversity within C. geophilum can be incredibly high, even at the scale of a single soil core. This highlights the need to characterize Cenococcum samples phylogenetically prior to population studies so that cryptic, reproductively isolated species are not admixed together in the analyses. Also sampling design and effort are critical for understanding population and phylogenetic diversity of C. geophilum. A recent population study targeted one Cenococcum lineage in Japanese pine forests and found no spatial autocorrelation at the forest stand level but did find evidence for a pattern of isolation by distance at larger spatial scales. These observations are consistent with the possibility of cryptic recombination. Another recent phylogenetic study found that several Cenococcum lineages are widely distributed across multiple regions and continents. This indicates that some lineages within C. geophilum may be ancient or that cryptic long-distance dispersal is ongoing. Overall, our assessment and review of the recent literature suggests that additional research is needed to understand the population structure and biology of C. geophilum.