Assembly of endosymbiont communities shaped by within-host species interactions

Background/Question/Methods

Endosymbionts represent a unique challenge to community assembly theory because of their inherent reliance on their host species’ assembly for their own persistence. Endosymbiont assembly is therefore a balance among (1) processes external to the host, (2) host-endosymbiont interactions specific to the host, and (3) endosymbiont-endosymbiont interactions within the host. Here, we test the relative contributions of these three levels using an experimental framework manipulating each level. We investigated how fungal endophyte assembly in the dunegrass Ammophila breviligulata is influenced by water stress, host genotype, and presence of the systemic fungal endophyte Epichlöe (E+ and E- treatments). Leaves of experimental plants were sampled in August and October 2011. Plant tissue was surface sterilized and plated on nutrient agar. Emergent fungi were identified through sequencing of the ITS region and subsequent GenBank BLAST searches.

Results/Conclusions

Epichlöe emerged in >90% of E+ plant tissue compared to 0% of E- tissue. The E+ treatment suppressed the frequency of other colonizing endophytes by >50% compared to the E- treatment. The water treatment and host genotype had relatively low effects on endophyte colonization. These results show that endophyte colonization is highly influenced by interactions among endophytes and less so by the surrounding environment and host genotype. Our results shed light on the processes important to endosymbiont assembly, and suggest that within-host competition for space or resources may drive the assembly of endophyte communities.