Defensive symbiosis and the evolution of virulence

Paul Nelson, Georgiana May

Research output: Contribution to journalArticlepeer-review

3 Scopus citations


A microbiome rife with enemies of the host should cause selection for defensive traits in symbionts, yet such complex environments are also predicted to select for greater symbiont virulence. Why then do we so often observe defensive mutualists that protect hosts while causing little to no damage? To address this question, we build a symbiont-centered model that incorporates the evolution of two independent symbiont traits: Defense and virulence. Virulence is modeled as a continuous trait spanning parasitism (positive virulence) and mutualism (negative virulence), thus accounting for the entire range of direct effects that symbionts have on host mortality. Defense is modeled as a continuous trait that ameliorates the costs to the host associated with infection by a deleterious parasite. We show that the evolution of increased defense in one symbiont may lead to the evolution of lower virulence in both symbionts and even facilitate pathogens evolving to mutualism. However, results are context dependent, and when defensive traits are costly, the evolution of greater defense may also lead to the evolution of greater virulence, breaking the common expectation that defensive symbionts are necessarily mutualists toward the host.

Original languageEnglish (US)
Pages (from-to)333-343
Number of pages11
JournalAmerican Naturalist
Issue number3
StatePublished - Sep 1 2020

Bibliographical note

Publisher Copyright:
© 2020 by The University of Chicago.


  • Defense
  • Microbiome
  • Mutualism
  • Parasitism
  • Symbiosis
  • Trade-offs
  • Plant Diseases/microbiology
  • Plants/microbiology
  • Virulence
  • Host-Pathogen Interactions/physiology
  • Symbiosis/physiology
  • Biological Evolution
  • Models, Biological

PubMed: MeSH publication types

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


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