Understanding factors that control the composition and distribution of communities of host-associated microbes has been a recent focus in microbial ecology. Over the past decade we have examined fungal communities in asymptomatic leaves of a broadly distributed endemic tree in the Hawaiian Islands, Metrosideros polymorpha, via high-throughput amplicon sequencing of the nuclear ribosomal internal transcribed spacer (ITS). Across a heterogeneous landscape and between islands, we found high alpha and beta diversity in these endophytic communities. Environmental factors including elevation and rainfall, and to a lesser extent geographic distance, explain variation in community structure. To frame questions regarding ecological and evolutionary processes underlying these patterns, we have focused on highly abundant clades identified in a phylogenetic context. In doing so we have identified a previously unknown, highly diverse clade in the family Teratospheriaceae (Ascomycota) that predominates in foliar communities at high elevation sites on the Island of Hawai'i. To understand the evolutionary context of this clade we sampled Metrosiderosin a common garden and in high elevation sites on the Island of Maui.
We used the Illumina MiSeq platform to sequence ITS (fungal) and 16S (bacterial) amplicons from leaves of these trees and used statistical and phylogenetic analyses to examine phylogeography in this focal clade. Evaluation of leaves from trees in high-elevation and common-garden settings revealed a strong association between abaxial foliar trichomes and members of this previously unknown clade, an observation confirmed by microscopy. The prevalence of this clade across multiple environments on different islands suggests a persistent ecological association with trichomes. In Metrosideros, the presence of leaf trichomes is thought to be a morphological adaptation that can reduce drought stress, temperature stress, and herbivory pressure. We have observed that individuals in this fungal clade grow extensively among and within foliar trichomes but not within photosynthetic leaf tissue. However, it is still not known what the ecological role of these fungi may be. Ongoing work using transcriptomic approaches will generate hypotheses regarding the mechanisms of interaction between host and fungus in these isolated alpine environments.