RUSSULA, A MODEL TO STUDY ECTOMYCORRHIZAL FUNGAL DIVERSITY, DISTRIBUTION, AND ENVIRONMENTAL THREATS

The genus Russula (Russulales) is one of the most taxonomically diverse and ecologically significant groups of ectomycorrhizal fungi, forming mutualistic associations with all major lineages of ectomycorrhizal plants worldwide. However, limited understanding of their species-level diversity, ecology, and distribution makes it difficult to assess threats to Russula persistence. This work explores threats to fungal diversity in spruce-fir forests of the southern Appalachians, along with the diversity, distribution, and evolutionary history of Russula at both local and global scales. It was determined that soil lead (Pb) concentrations were negatively associated with ectomycorrhizal fungal diversity, including Russula in spruce-fir forests of the southern Appalachians. In this region, 30 species-level Russula clades were identified by utilizing a three-pronged approach that included fresh field collections, the identification of Russula OTUs from environmental sequencing efforts, and a revision of herbarium specimens. Soil manganese (Mn) concentration was the strongest factor influencing Russula occurrence, with 46% of the species analyzed showing a decline with increasing soil Mn concentrations. Individual Russula species occurrences were shaped by a variety of edaphic factors, highlighting their diverse ecological roles in the spruce-fir ecosystem. A global multilocus phylogenetic analysis of Russula subsection Xerampelinae revealed at least 23 species, including three new species: R. lapponica, R. neopascua, and R. olympiana. While some species in the subsection were distributed across the Holartic region, others were geographically restricted. Most species were found to be host generalists, with their evolutionary history likely influenced by the distribution and diversification of their primary hosts. This study highlights the significant impact of edaphic factors on Russula diversity. It also provides the first multilocus phylogenetic framework for Russula subsection Xerampelinae, revealing patterns of species diversity, host associations, and biogeographic distributions. These findings contribute to a deeper understanding of the ecological roles and evolutionary history of Russula, while highlighting broad threats to it and other ectomycorrhizal diversity.