Evolution of transmission mode in obligate symbionts

Devin M. Drown, Peter C. Zee, Yaniv Brandvain, Michael J. Wade

Research output: Contribution to journalArticlepeer-review

21 Scopus citations


Background: A host obtains symbionts by horizontal transmission when infected from the environment or contagiously from other hosts in the same generation. In contrast, vertical transmission occurs when a host obtains its symbionts directly from its parents. Either vertical or horizontal transmission can sustain an association between a host and its symbiont. Questions: What evolutionary forces are necessary to evolve from an ancestral state of horizontal transmission to a derived state of vertical transmission? Mathematical methods: We explore a general model of fitness interaction, including both additive and epistatic effects, between host and symbiont genes. Recursion equations allow us to analyse the short-term behaviour of the model and to study long-term deterministic effects with numerical iterations. Key assumptions: Obligate interaction between a symbiont and a single host species with genetically determined horizontal and vertical transmission. No free-living symbionts or uninfected hosts and each host is infected by only a single symbiont genetic lineage (no multiple infections). No population structure. Conclusions: Epistasis for fitness between host and symbiont genes, like that in a matching alleles model, is a necessary condition for the evolution of vertical from horizontal transmission. Stochastic individual-based simulations show that (1) mutation facilitates the switch to vertical transmission and (2) vertical transmission is a stable evolutionary endpoint for a matching alleles model.

Original languageEnglish (US)
Pages (from-to)43-59
Number of pages17
JournalEvolutionary Ecology Research
Issue number1
StatePublished - 2013


  • Co-evolution
  • Epistasis
  • Horizontal transmission
  • Linkage disequilibrium
  • Vertical transmission


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