Do Interactions among Microbial Symbionts Cause Selection for Greater Pathogen Virulence?

Research output: Contribution to journalArticlepeer-review


The ecological and evolutionary consequences of microbiome treatments aimed at protecting plants and animals against infectious disease are not well understood, even as such biological control measures become more common in agriculture and medicine. Notably, we lack information on the impacts of symbionts on pathogen fitness with which to project the consequences of competition for the evolution of virulence. To address this gap, we estimated fitness consequences for a common plant pathogen, Ustilago maydis, over differing virulence levels and when the host plant (Zea mays) is coinfected with a defensive symbiont (Fusarium verticillioides) and compared these fitness estimates to those obtained when the symbiont is absent. Here, virulence is measured as the reduction in the growth of the host caused by pathogen infection. Results of aster statistical models demonstrate that the defensive symbiont most negatively affects pathogen infection and that these effects propagate through subsequent stages of disease development to cause lower pathogen fitness across all virulence levels. Moreover, the virulence level at which pathogen fitness is maximal is higher in the presence of the defensive symbiont than in its absence. Thus, as expected from theory for multiple parasites, competition from the defensive symbiont may cause selection for increased pathogen virulence. More broadly, we consider that the evolutionary impacts of interactions between pathogens and microbial symbionts will depend critically on biological context and environment and that interactions among diverse microbial symbionts in spatially heterogeneous communities contribute to the maintenance of the highly diverse symbiotic functions observed in these communities.

Original languageEnglish (US)
Pages (from-to)252-265
Number of pages14
JournalAmerican Naturalist
Issue number2
StatePublished - Feb 2022

Bibliographical note

Funding Information:
We thank Keunsub Lee for tireless and careful work in his dissertation research (partially supported by National Science Foundation [NSF] grant DEB-0723451 to G.M.). The aster analyses presented here were supported by NSF grant DEB-1046065 to G.M. and were conducted by coauthors D.J.E. and C.J.G. We thank two anonymous reviewers for thoughtful and constructive comments and American Naturalist editors for careful consideration of the manuscript and constructive suggestions and edits. G.M. and R.G.S. acknowledge continuing inspiration of the University of Minnesota Center for Community Genetics.

Publisher Copyright:
© 2022 The University of Chicago.


  • Aster analysis
  • Defensive symbiosis
  • Disease control
  • Microbiome
  • Parasites
  • Virulence evolution

PubMed: MeSH publication types

  • Journal Article
  • Research Support, U.S. Gov't, Non-P.H.S.


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