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Updated: May 1, 2026

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Published on: April 2, 2013
Evolution of transmission mode in obligate symbionts
Devin M Drown1, Peter C Zee1, Yaniv Brandvain1
1Department of Biology, Indiana University, Bloomington, Indiana, USA.
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.
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