Foliar endophytic fungi diverge from forest-scale island biogeography patterns observed in ectomycorrhizae of Pinus spp

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Arik Joukhajian, Microbiology and Plant Pathology, UC Riverside, Riverside, CA, A. Elizabeth Arnold, School of Plant Sciences and Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ and Sydney I. Glassman, Microbiology and Plant Pathology, University of California, Riverside, Riverside, CA

Background/Question/Methods
Microbial community diversity can adhere to the principles of island biogeography, such that
diversity correlates with island size and age, and isolation restricts immigration. By modeling
systems in this context, we can better understand drivers of ecological diversity at multiple
scales. Prior studies show that ectomycorrhizal fungi, which form mutualistic associations with
tree roots, adhere to island biogeography principles when trees that are isolated from a forest
edge are treated as islands. Foliar endophytic fungi are symbiotic inhabitants of leaves, but
whether they similarly reflect island biogeography principles at tree- and leaf scales is not
known. We used Illumina MiSeq sequencing of the internal transcribed spacer (ITS1) region to
examine foliar endophytic fungal communities in healthy needles of 40 isolated pine trees (Pinus
albicaulis, P. contorta) in a subalpine basin in Yosemite National Park. In parallel, we used
Sanger sequencing to identify endophytic fungi isolated in culture. We evaluated variation in
endophyte diversity and community structure as a function of tree size, tree age, and needle
age for each tree species, and characterized the taxonomy of endophytes in this relatively
extreme environment for isolated trees.
Results/Conclusions
The richness of endophytic fungi per tree was low relative to that of pines sampled with similar
methods in other environments, with a median of 9 species per individual. However, high
variation in richness was observed in Pinus albicaulis, with some individuals harboring more
than four times that median richness. As a whole, the community was species-rich, with more
than 270 putative species occurring in foliage of these isolated trees. Illumina sequencing and
culturing approaches were complementary, with one family unique to cultures and 64 families
unique to Illumina MiSeq. Overall the dominant class was Dothideomycetes (Ascomycota),
represented primarily by the endophyte-rich order Capnodiales. Richness was significantly
higher in older needles in P. albicaulis, but not in P. contorta. Unlike ectomycorrhizal fungi in the
same trees, foliar endophyte richness was not correlated strongly with tree age or tree size. We
concluded that foliar endophytic fungi do not adhere simply to the island biogeography model
with respect to the tree as a whole, perhaps reflecting their airborne dispersal. They merit
further study at a finer spatial scale to determine the ecological processes shaping their
communities.