title: Evolution of transmission mode in obligate symbionts.
abstract: 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.
  
  
  OBJECTIVE
  What evolutionary forces are necessary to evolve from an ancestral state of horizontal transmission to a derived state of vertical transmission?
  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.
  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.
authors.affiliation: Department of Biology, Indiana University, Bloomington, Indiana, USA.
authors.forename: Devin M
authors.initials: DM
authors.lastname: Drown
authors.affiliation: Department of Biology, Indiana University, Bloomington, Indiana, USA.
authors.forename: Peter C
authors.initials: PC
authors.lastname: Zee
authors.affiliation: Department of Biology, Indiana University, Bloomington, Indiana, USA.
authors.forename: Yaniv
authors.initials: Y
authors.lastname: Brandvain
authors.affiliation: Department of Biology, Indiana University, Bloomington, Indiana, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Wade
keywords: co-evolution; epistasis; horizontal transmission; linkage disequilibrium; vertical transmission