Phylogeny and diversity of Rhizopogon species in Japan

Rhizopogon (Boletales) is an ectomycorrhizal (ECM) fungal genus that forms specific symbiotic associations with host trees within the family Pinaceae, playing a crucial role in seedling growth and establishment. Despite its ecological importance, the species diversity and phylogenetic relationships of Rhizopogon in East Asia remain poorly understood. Here, we investigate the diversity, phylogeny, and host specificity of Japanese Rhizopogon species. We conducted phylogenetic analyses of 106 basidiomata and three ECM root tip samples collected across Japan using three loci: the internal transcribed spacer (ITS), large subunit ribosomal RNA (LSU), and RNA polymerase II subunit B (RPB2). Phylogenetic analyses revealed at least 16 distinct lineages, including one only detected from a single root tip collected in Japan. Among the lineages with basidiomata, five corresponded to previously described species (R. alpinus, R. boninensis, R. lucidus, R. togasawarius, and R. yakushimensis). Seven other lineages, which did not cluster with any previously recognized species, are described here as new species and detailed morphological characteristics are provided: R. citrinus, R. filivolubilis, R. flavoroseolus, R. japonicus, R. melliodorus, R. margaritaceus, and R. politus. Two other lineages are new to Japan: R. pseudoroseolus and R. evadens. Another lineage, previously regarded as the Japanese “Shouro,” was reidentified as R. graveolens based on morphological and molecular evidence. This study is the first comprehensive taxonomic assessment of Rhizopogon in East Asia.