General Trends of Genomic Signatures of the Ectomycorrhizal Symbiosis in Fungi: A Comparison Across Multiple Lineages

Ectomycorrhizal (ECM) symbiosis is a mutualistic association between the roots of plants, mainly tree species, and fungi, where the photosynthetic partner provides carbon to the fungus in exchange for nutrients, such as nitrogen and phosphorous. Because the majority of forest trees form this type of association, they have a tremendous impact on the maintenance and functioning of forest ecosystems. This symbiosis evolved multiple times in fungi from diverse saprotrophic ancestors. Comparative genomic studies have shown general trends in the evolution of ECM fungi, including the loss of plant cell wall degrading enzymes (PCWDEs), expansion of transposable elements and small secreted proteins, and the co-option of genes already present in saprotrophic ancestors. However, each independently evolved ECM lineage has adopted different ways of interacting with their hosts by recruiting different sets of ancestral gene families for symbiotic functions and retaining distinct sets of PCWDEs, depending on the ecology and decomposing abilities of their ancestors. Here, we review the main findings in the field of ECM fungal genomics and present the main differences among various taxonomic groups. Finally, we provide insights into new frontiers in this field.