COS 5-7
Experimental evidence for environmental and biotic control of beach grass endosymbiont communities
All macrooorganims are host to a large diversity of microbial endosymbionts. Recent technological advances have allowed for the characterization of many communities of endosymbionts, yet much of their ecology, and in particular how endosymbionts assemble into communities, remains little understood. Endosymbiont communities are likely influenced by four general factors: the environment external to their host, the host species, interactions among endosymbionts within a host, and spatial factors such as dispersal. With the goal of quantifying the relative influence of these factors, we conducted a reciprocal transplant experiment focusing on the assembly of fungal endophytes (fungi living asymptomatically within hosts) in roots of beach grasses along the USA Northwest Coast. We collected tillers of three common grass species (Elymus mollis, Ammophila arenaria, A. breviligulata) on dunes with preexisting root endophyte communities from two environmentally-distinct sources, the foredune and backdune. Using different sources of tillers allowed us to quantify effects of existing endophyte communities on subsequent colonization. We reciprocally transplanted the tillers into control or host-removal plots designed to reduce endophyte dispersal from nearby roots in the foredune and backdune. We cultured endophytes from the roots of experimental plants prior to transplanting, and again at the conclusion of the experiment. We sequenced the ITS region of all emergent fungi, and clustered the sequences based on 97% similarity.
Results/Conclusions
We found evidence for multiple interactive controls of endophyte communities, most notably the environment into which the tiller was transplanted. Backdune transplants had a higher frequency of endophyte isolations than foredune transplants regardless of source, suggesting a strong role of environment on overall endophyte abundance. Endophyte colonization differed across host species, with increases in endophyte isolation frequency in the two Ammophila species and a decrease in E. mollis. There was a significant interaction between tiller source and environment on endophyte communities, indicating that communities are shaped by both the endophytes in the environment as well as those endophytes already present within a host. The host-removal treatment had little overall effect on endophyte communities but did increase the isolation frequency of the most common OTU. Overall, we conclude that the environment external to the host is likely the most important factor in shaping endophyte communities, but that host species, interactions among endophytes, and dispersal also play important roles. Disentangling the complexity of endosymbiont community assembly contributes to a clearer understanding of endosymbiont communities and their diversity.