Aquatic hyphomycete fungi are important in streams because they make carbon from plant debris available to the stream food web. Some species, including species in the genus Tetracladium, are multifunctional, and are also endobionts of plants on land. Tetracladium spp. are reported from plants and soils world-wide, e.g. wheat in Sweden and mosses in Antarctica. In spite of their presence in crops, extreme/fragile environments, and threatened plants, we know extremely little about their biology on land. Combining classic field and experimental work with cutting-edge -omics sequencing, I will address this knowledge gap by asking three questions. 1) How are populations on land and in water connected? Using genome-wide SNP data for ~600 individuals I will determine how genotypic diversity is structured or shared between habitats. 2) How are two ecologies achieved from one genome? By comparing the transcriptomes of saprotrophs and endobionts I will uncover how gene expression is differentially regulated in these two lifestyles. 3). How do they impact plants? Using co-culture of fungi with plants, I will study the fungal impact on above and below ground biomass production in plants. The results of this study will greatly expand our extremely limited understanding of Tetracladium spp. in their terrestrial roles, provide needed data to understand their impact on plant hosts, and open a new system to study fungal evolution and plant-microbe interactions.