Roles of environmental gradients and host identity in structuring fungal endophyte communities

Background/Question/Methods

Endosymbionts living within host organisms represent a large portion of the world’s diversity, yet their ecology remains relatively unexplored. In particular, there is little understanding of the mechanisms by which endosymbionts assemble into communities within their hosts. Previous studies of endosymbionts show patterns of diversity and distribution are influenced by the identity of the host and its external environment, yet the relative influence of these two factors on community assembly remains unclear. To examine how environment and host identity contribute to endosymbiont assembly, we sampled communities of asymptomatic fungal endophytes growing within three US Northwest coastal dune grasses (Ammophila arenaria, A. breviligulata, and Elymus mollis). Coastal dunes provide a natural environmental gradient consisting of a harsh foredune that gives way to a more stable backdune habitat. We used a culture-based approach to investigate how endophyte communities in the leaves and roots differ by host species and position along the dune gradient. 

Results/Conclusions

Environment strongly influenced leaf and root endophyte communities, as endophyte operational taxonomic unit (OTU) richness increased from the foredune to the backdune. Host species also significantly affected OTU richness (A. breviligulata < Ammophila arenaria < E. mollis), but this effect was considerably weaker than the effect of environment. Furthermore, host species shared a large proportion of endophytes suggesting that endophytes form generalist associations with hosts. Host roots harbored greater richness and total abundance of endophytes compared to leaves. We conclude that the environment is a stronger influence than host species in shaping endophyte communities in coastal dune grasses. This finding more generally suggests that endosymbiont community assembly may depend more on environmental factors and less on host identity.