Within the framework of the JGI Mycorrhizal Genomics Initiative, we are sequencing a phylogenetically and ecologically diverse suite of mycorrhizal fungi (Basidiomycota and Ascomycota), which include the major clades of symbiotic species associating with trees and woody shrubs. Analyses of these genomes will provide insight into the diversity of mechanisms for the mycorrhizal symbiosis, including ericoid-, orchid- and ectomycorrhizal associations.
Cenococcum geophilum is an ascomycetous fungus placed into the Dothideomycetes, where it represents the only known ectomycorrhizal species within this large and ecologically diverse class of Ascomycota. C. geophilum is one of the most common and globally abundant genera of ectomycorrhizal fungi, forming black ectomycorrhizas with darkly pigmented, stringy hyphae emanating from root tips (Figure 1). It has a broad host- and habitat range and is often the dominant mycorrhizal fungus on the tree-root systems in forests of arctic, temperate and subtropical environments. Therefore, understanding its ecological role in forest ecosystems is of great significance.
One of the important functions of the ectomycorrhizal symbiosis is an enhanced water uptake and drought resistance for host trees. Ectomycorrhizal tips of C. geophilum are strikingly abundant during drought when other mycorrhizal species decline. C. geophilum is highly resistant to desiccation. Genome-related functional studies will help elucidate whether and how the superior survival of C. geophilum under drought stress confers benefits to the host trees in water or nutrient uptake.
Although being ubiquitous, the biology of C. geophilum is poorly understood. It forms sclerotia as resistant propagules (Figure 2), but no definitive sexual or asexual spore-producing structures are known. Studies of fine-scale diversity of C. geophilum populations, however, revealed a high level of genetic polymorphism amongst individuals consistent with the occurrence of recombination mechanisms and suggesting that the fungus is reproducing sexually in nature. The genomic sequence will help understand the reproductive biology of the genus and will make it an excellent candidate for population genomic studies on a global scale.
geophilum strain 1.58 was isolated from a sclerotium in April
2008. It was collected in an irrigation experimental site in the
Pfyn forest which is situated in a dry inner alpine valley in
Switzerland (canton of Valais; 46.303 N, 7.613 E).
The genomic sequence of C. geophilum will also provide insights in the evolution of the ectomycorrhizal symbiosis in Dothideomycetes by comparing the presence of symbiosis-related genes found in other ectomycorrhizal ascomycetes, such as Tuber and Terfezia species. In addition, the genomic sequence of C. geophilum will contribute to a better prospect of mycorrhizal symbiosis as a fungal nutritional mode, which apparently harbors more than only one ecological strategy.