Breakthroughs in DNA sequencing have upended our understanding of fungal diversity. Only ∼155,000 of the 2-3 million fungal species on the planet have been formally described and named, and 'dark taxa' - species known only from sequences - represent the vast majority of species within the fungal kingdom. The International Code of Nomenclature requires physical type specimens to officially recognize new fungal species, making it difficult to name dark taxa. This is a significant problem for conservation because, without names, species cannot be recognized for environmental and legal protection. Symbiotic ectomycorrhizal (EcM) fungi play a particularly important role in forest carbon drawdown, but at present we have little understanding of how many EcM fungal species exist, or where to prioritize research activities to survey and describe EcM fungal lineages. In this review, we use global soil metabarcoding databases (GlobalFungi and the Global Soil Mycobiome consortium) to evaluate current estimates of the total number of EcM fungal species on Earth, outline the current state of undescribed EcM dark taxa, and identify priority regions for future dark taxa exploration. The metabarcoding databases include up to 219,730 EcM fungal operational taxonomic units (OTUs) detected from almost 39,500 samples. Using Chao richness estimates corrected for extrapolating species numbers from metabarcoding datasets, we predict that the global diversity of EcM fungi could be ∼25,500-55,500 species. Dark taxa - those that do not match species-level identities - account for 79-83% of OTUs. Oceania contains the highest percentage of dark taxa (87%), and Europe the lowest (78%). Priority 'darkspots' for future research occur predominantly in tropical regions, but also in selected temperate forests at both southern and northern latitudes. We propose concrete steps to reduce the prevalence of EcM darkspots, including performing targeted field surveys, barcoding fungaria voucher specimens, and developing new ways to describe and conserve fungal taxa from DNA alone.

Department of Environmental Systems Science Institute of Integrative Biology ETH Zürich Zürich Switzerland

Laboratory of Environmental Microbiology Institute of Microbiology of the Czech Academy of Sciences Prague Czech Republic

Society for the Protection of Underground Networks Dover DE USA

Society for the Protection of Underground Networks Dover DE USA; Department of Environmental Systems Science Institute of Integrative Biology ETH Zürich Zürich Switzerland

Society for the Protection of Underground Networks Dover DE USA; Royal Botanic Gardens Victoria Melbourne VIC Australia; School of BioSciences University of Melbourne Parkville VIC 3010 Australia

Society for the Protection of Underground Networks Section Ecology and Evolution Vrije Universiteit Amsterdam Amsterdam The Netherlands

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