Abstract
Background and Aims
Mutualistic microbes often increase their host’s susceptibility to parasite infection, but the mechanism underlying this pattern remains unknown.
We tested two competing hypotheses to identify the cause of this phenomenon.
First, mutualist-provided resources could attract antagonists by making hosts more resource-rich.
Second, mutualism establishment itself might increase host vulnerability to antagonists.
Methods
To test which mechanism underlies increased parasite susceptibility, we experimentally decoupled mutualism establishment and mutualist-provided resources in the legume-rhizobia mutualism.
We measured parasite load on Medicago truncatula plants infected with root-knot nematodes in a full-factorial design, in which we independently manipulated rhizobia nodulation (mutualism establishment) and nitrogen availability (mutualist-provided resources).
Key Results
We found little effect of mutualism establishment on susceptibility, as nodulation without nitrogen fixation did not significantly increase parasite infection.
By contrast, nitrogen did increase parasite infection. However, its effect was non-linear and was not explained by nitrogen assimilation into plant tissues, indicating that this effect is not driven by parasite attraction to resource-rich hosts.
Conclusions
Our results suggest that mutualist-provided resources are an important driver of indirect ecological costs of mutualism, although the mechanism linking mutualist-provided resources and susceptibility to infection remains unknown.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Statement of authorship: EY and CWW designed the study, EY and NM ran the experiment and collected data, CWW and EY analyzed data, EY wrote the first draft of the manuscript, and all authors contributed to revisions.
Data accessibility statement: The data and code supporting results have been archived on Figshare. https://doi.org/10.6084/m9.figshare.24442738
Discussion revised; p-values added directly to figures; nitrogen and rhizobia strains corrected (5 mM was formerly listed as 2.5 mM nitrogen, while USDA1021 and WSM1022 were designated as Em1021 and Em1022); Supplemental files updated
