A molecular contribution to the assessment of the Tricholoma equestre species complex

In recent years, interest in the Tricholoma equestre species complex has increased because of several cases of severe and sometimes fatal rhabdomyolysis reported in France and Poland. These occurred after repeated consumption of large portions of T. equestre sporophores during consecutive meals, despite the fact that this species is renowned as a tasty edible wild mushroom. The T. equestre species complex includes three ectomycorrhizal species Tricholoma flavovirens (Pers.) S. Lundell, Tricholoma auratum (Paulet) Gillet, and T. equestre (L.) P. Kummer. All these species produce sporophores with intense yellow gills but are difficult to distinguish by morphological analyses at both the macroscopic and microscopic levels. In T. equestre, two additional varieties are recognized: T. equestre var. populinum (Christensen & Noordeloos) associated with Populus sp. and/or Betula sp. trees and sometimes recognized as Tricholoma frondosae (Kalamees & Shchukin) and T. equestre var. pallidifolia characterized by pale to white gills, frequently recognized as Tricholoma joachimii (Bon & Riva). To explore the taxonomic (species delimitation), ecological, and geographical extent and limits of the T. equestre species complex, we have carried out a molecular comparison of worldwide strains belonging to this complex by using sequences of two molecular markers: the internal transcript spacer (ITS)1/5.8S/ITS2 region of the nuclear ribosomal unit and the 5' part of the mitochondrial cox1 gene. Phylogenetic analyses support the placement of European T. equestre, T. flavovirens, and T. auratum strains as representatives of a single species. This species appears associated with various conifers trees, depending on the geographic origin (Pinus pinaster for T. auratum, Pinus sylvestris or Abies alba for T. equestre and T. flavovirens). However, in the context of a single T. equestre species, the geographical location could lead to the characterization of sub-species or varieties, as suggested by the gathering of the four Asian (Japanese) T. auratum strains in a strongly supported distinct phylogenetic clade. Moreover, our analysis strongly argues for considering T. joachimii and the synonymised T. equestre var. pallidifolia as two representatives of a different species not belonging to the T. equestre group. This species would be phylogenetically related to the Tricholoma columbetta species with which they share white gills. Similarly, the phylogenetic analysis of the molecular data and the lack of gene flow between the strains associated with broad-leaved trees and those of the T. equestre complex, rather argues for two distinct species depending on the ecological niche: T. frondosae under broad-leaved trees and T. equestre under conifers.