Pyrosequencing of tropical fungal endophytes provides evidence for adaptation to high elevation

Background/Question/Methods

Fungal endophytes, the fungi that inhabit the asymptomatic leaves of all plants surveyed to date, represent a large and unknown component of global fungal diversity. Previous studies have shown them to exhibit a diverse range of functional roles in ecosystems, including protecting their plant host from herbivory or pathogen damage, reducing host nutrient and/or water stress, and as latent decomposers. Yet, we have very little understanding of the full extent of their diversity, especially in tropical forests, where they have been shown to be especially speciose. Further, there have been few studies that explore the relationship between these cryptic organisms and their broader environment, particularly the extent to which novel habitats could contribute to adaptive radiation in this class of organisms.  To assess the extent to which adaptive radiation and selection for novel habitats influence the structuring of these diverse fungal communities, we quantified endophtye communities in the leaves of a single host (Metrosideros polymorpha) on a single substrate across a broad elevational gradient (100 - 2400 m) on Mauna Loa Volcano, Hawaii using deep barcoded amplicon pyrosequencing (>600,000 sequences in total).  To assess the extent of fine level (within-genus and within-species) sequence variation in endophyte communities spanning this gradient, we used the highly-variable ITS1 nrDNA region as a proxy for evolutionary divergence. 

Results/Conclusions

We found very high endophyte richness across the landscape (more than 4,200 OTUs).  However, we also found that while fungal endophytic communities at the lowest elevation site (100 m) are significantly richer than the other sites at or above the species level (<95% ITS1 similarity), high elevation sites (1800 and 2400 m) show evidence for significantly higher levels of ITS1 sequence diversity at OTU similarity levels >95% (corresponding roughly to the species boundary in most fungi).  We also find that ITS1 sequence GC-content increases significantly from low to high elevation sites, suggesting selection for organisms better able to tolerate the higher levels of UV exposure and temperature extremes that organisms must face at those elevations.  This work describes evidence for yet another example of adaptive radiation in the Hawaiian Islands, albeit in a novel group of organisms and with a novel method, and suggests that the incredibly high diversity of these microfungi within this single host species may be in part due to their in situ diversification across a heterogenous landscape.