Basidiomycetes on Ferns

Fungi are ubiquitous in their associations with plants as biotrophs, necrotrophs, saprotrophs, mutualists and commensals. Plant pathogens and mycorrhizal symbionts of seed plants have received significant attention due to the importance of the hosts and the impact of the fungal symbionts. However, fungal relationships with non-seed plants have received less attention. Given the hypothesized role of fungi in the transition of plants from aquatic to terrestrial environments, the association between non-seed plants and fungi have the potential to represent some of the earliest symbioses between fungi and land plants and may provide unique insight into their evolutionary significance. Prerequisite to any understanding of the ecological and evolutionary role of these symbioses is an improved understanding of the distribution and diversity of fungi associated with non-seed plants. Ferns are known to have extensive relationships with endophytic Mucoromycotina and Glomeromycota, act as hosts to rust fungi, and also harbor a diversity of ascomycetes. However, outside of decay fungi on senescent stems and leaves, relationships between ferns and basidiomycetes are essentially unheard of. We report here the first exception, a resupinate basidiomycete that forms conspicuous long-lived colonies over the leaves of 14 species in a clade of the fern genus Elaphoglossum, a prominent epiphytic genus of tropical humid forests. For each host fern species, these basidiomycetes were present on all leaves of all plants in the forested areas where they were found growing. Plants showed no sign of necrosis, thus we consider this to be an epibiotic relationship. Using molecular DNA sequences (nucITS, LSU, mtSSU, RPB1, RPB2, SSU) of 164 fungal species, we identify the fungi as a member of the Trechisporales (Agaricomyces), a clade otherwise comprising resupinate and crust forming saprophytes. Our samples form a clade distinct from all other members of the order and require us to coin a new genus. We also reconstruct a host plant phylogeny to test the hypotheses about the evolution of the relationship