Fungals in the Jungles: Endophytic fungal diversity at the landscape scale

Background/Question/Methods

Fungal endophytes are microfungi that inhabit the asymptomatic aboveground tissues of their host plants. Most work examining the effects of endophytic fungi on their hosts is focused on agricultural grasses; relatively little is known about endophytes in woody vegetation – especially in the tropics, where the few extant studies show endophytes to be ubiquitous and hyperdiverse.  In order to better understand the patterns of endophyte distribution at the landscape scale, we selected a model host within a model environmental system. Metrosideros polymorpha, a tree endemic to Hawaiʻi, occurs across an extraordinary range of elevation and rainfall. The Mauna Loa environmental matrix on the Island of Hawaiʻi includes Metrosideros-dominated ecosystems that vary in mean annual temperature from 10–23° C and mean annual precipitation from 500 to 5500 mm, while holding constant the physical and chemical properties of the soil matrix. Barcoded pyrosequencing of the ITS1 region was used to group fungi into Operational Taxonomic Units (OTUs) and quantify community composition in surface-sterilized leaves from trees within a matrix of sites across the landscape.

Results/Conclusions

Sequencing resulted in over 700,000 high-quality sequences from 130+ trees, with an average of over 5,000 fungal sequences per tree.  We found very high levels of both alpha and beta diversity across the environmental matrix.  Grouping sequences into OTUs at 95% similarity, per-tree richness ranged from 30 to over 200, per-site from 500 to over 800 OTUs, and overall fungal richness across the landscape was more than 4,100.  This is much greater than previous studies have shown, particularly when considering that all fungi were resident on a single host species at sites within 100 km of each other.  Nonmetric multidimensional scaling showed that community similarity is strongly correlated with environmental variables including temperature/elevation and rainfall.  Mantel correlograms suggest positive spatial autocorrelation is important for community similarity between sites up to 20 km from each other.  We argue that these results imply that endophytic fungal diversity may be much greater than previously thought, because even communities on a single host may vary greatly across a landscape, depending on external environmental conditions.