Taxonomic revision of the Japanese Tricholoma ustale and closely related species based on molecular phylogenetic and morphological data

“Kakishimeji” identified as Tricholoma ustale and belonging to Tricholoma sect. Genuina is a common poisonous mushroom in Japan. Kakishimeji contains the toxic compound ustalic acid and causes digestive trouble. However, this fungus is consumed in some regions of Japan without any digestive issues. We clarified the probable species complex of Kakishimeji based on a phylogenetic analysis. We collected 89 basidioma specimens of Kakishimeji and related species from various forest sites in Japan and conducted phylogenetic analyses using 7 nuclear and mitochondrial gene sequences. Kakishimeji was found to consist of four distinct phylogenetic clades based on all DNA regions tested. Of these, two clades included European T. stans and T. albobrunneum type specimens. Another two clades consisted of sister clades to T. pessundatum and T. ustaloides. In addition, all four phylogenetic clades of Kakishimeji had different spore and basidium sizes. Therefore, we regarded the latter two clades as two new Tricholoma species: T. kakishimeji and T. kakishimejioides.