Abstract
Suillus grevillei is an ectomycorrhizal fungus of larch that occurs widely throughout the Northern Hemisphere. According to its pileus color and geographic distributions, two closely related species are recognized: European S. grevillei that has a yellowish pileus and North American S. clintonianus that has a reddish pileus. Because specimens in eastern Eurasia are understudied, we incorporated basidioma collections from Japan and eastern Siberia to clarify the taxonomy and distribution of the S. grevillei complex in a global context. The multi-locus phylogeny assembled with four conservative loci (LSU, RPB1, RPB2, and TEF) revealed three monophyletic clades with strong branching support, one of which is described as a new species, S. orientalis. Morphologically, S. orientalis is distinguished from S. grevillei by its dark reddish-brown pileus and from S. clintonianus by having smaller spores and the absence of encrusted hyphae in the pileipellis. S. orientalis is distributed in eastern Eurasia, S. grevillei in Europe and eastern Asia, and S. clintonianus in North America. The multi-locus phylogeny of conservative DNA markers was more effective than rapidly evolving and highly variable ITS regions in recognizing closely related species within the S. grevillei complex.