Molecular Systematics and Evolution of Fungi in the Russuloid Lineage

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9974018Miller Species in the mushroom genera Russula and Lactarius are conspicuous abundant elements of temperate zone forests and arctic alpine ecosystems, and increasingly are known as important elements in tropical regions of the world. They contribute significantly to fungal biomass, are important dietary elements for rodents and insects, and many species are harvested worldwide for human consumption. As root symbionts (ectomycorrhizae) with forest trees, species of Russula may constitute nearly a quarter of the total mycorrhizal flora in various temperate and tropical forests. Despite their conspicuous and cosmopolitan nature, species in both genera have been difficult to identify and to characterize. Taxonomy of the group has relied on features that are known to exhibit phenotypic plasticity (intergrading color variation, for example, and intergrading spore shape) or suspected of arising in parallel in species groups (shape of the fruiting body, for example). In addition, the study of tropical taxa has considerably altered earlier taxonomic ideas that were principally based on a restricted sampling of fungi from temperate zones of the northern hemisphere. The number of described species is now large, but all major classifications are regional in nature, and fail to provide a comprehensive overview of the genera on a worldwide basis. New molecular evidence from DNA sequencing of nuclear ribosomal genes shows promise of yielding data useful for constructing a phylogenetic framework of evolutionary relationships among the species groups of Russula and Lactarius and numerous allies in the basidiomycetes. Dr. Steven Miller of the University of Wyoming is working with an impressive network of collaborators in several countries to collect samples of these fungi and analyze them for DNA sequence diversity in the nuclear ribosomal genes, and to integrate these data with new and traditional morphological investigations. In particular, contacts and colleagues in South America, in Africa, and in Australasia will allow much-needed sampling of tropical representatives of the genera to balance temperate-zone exemplars, in the molecular and morphological studies. Preliminary studies have been accomplished on ca. 80 taxa from the order Russulales, demonstrating feasibility of the project and promising resolution from the DNA data. Well supported lineages emerging from the molecular-phylogenetic analyses will become the focus of intensive morphological studies to characterize reliable features for species identification. Another direct result of the newly developed classification and gene sequence data will be the capability molecularly to identify these fungi on individual mycorrhizal roots even in the absence of fruit-body production, thus facilitating ecological research in temperate and tropical ecosystems.