Evol Ecol Res 15: 43-59 (2013)     Full PDF if your library subscribes.

Evolution of transmission mode in obligate symbionts

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

Department of Biology, Indiana University, Bloomington, Indiana, USA

Correspondence: D.M. Drown, 1001 E Third Street, Indiana University, Bloomington, IN 47405, USA. E-mail: dmdrown@indiana.edu


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.

Keywords: co-evolution, epistasis, horizontal transmission, linkage disequilibrium, vertical transmission.

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