title: The spread of incompatibility-inducing parasites in sub-divided host populations.
abstract: BACKGROUND
  Maternally transmitted symbionts have evolved a variety of ways to promote their spread through host populations. One strategy is to hamper the reproduction of uninfected females by a mechanism called cytoplasmic incompatibility (CI). CI occurs in crosses between infected males and uninfected females and leads to partial to near-complete infertility. CI-infections are under positive frequency-dependent selection and require genetic drift to overcome the range of low frequencies where they are counter-selected. Given the importance of drift, population sub-division would be expected to facilitate the spread of CI. Nevertheless, a previous model concluded that variance in infection between competing groups of breeding individuals impedes the spread of CI.
  
  
  RESULTS
  In this paper we derive a model on the spread of CI-infections in populations composed of demes linked by restricted migration. Our model shows that population sub-division facilitates the invasion of CI. While host philopatry (low migration) favours the spread of infection, deme size has a non-monotonous effect, with CI-invasion being most likely at intermediate deme size. Individual-based simulations confirm these predictions and show that high levels of local drift speed up invasion but prevent high levels of prevalence across the entire population. Additional simulations with sex-specific migration rates further show that low migration rates of both sexes are required to facilitate the spread of CI.
  
  
  CONCLUSIONS
  Our analyses show that population structure facilitates the invasion of CI-infections. Since some level of sub-division is likely to occur in most natural populations, our results help to explain the high incidence of CI-infections across species of arthropods. Furthermore, our work has important implications for the use of CI-systems in order to genetically modify natural populations of disease vectors.
authors.affiliation: Research Department for Genetics, Evolution and Environment, Faculty of Life Sciences, University College London, Wolfson House, 4 Stephenson Way, London, NW1 2HE, UK. m.reuter@ucl.ac.uk
authors.forename: Max
authors.initials: M
authors.lastname: Reuter
authors.forename: Laurent
authors.initials: L
authors.lastname: Lehmann
authors.forename: Frédéric
authors.initials: F
authors.lastname: Guillaume
mesh_terms: D000818:Animals; D003198:Computer Simulation; D003593:Cytoplasm / Q000235:genetics / Q000469:parasitology*; D005260:Female; D006790:Host-Parasite Interactions / Q000235:genetics*; D018445:Infectious Disease Transmission, Vertical; D008018:Life Cycle Stages; D008297:Male; D008957:Models, Genetic; D011157:Population Dynamics; D013559:Symbiosis / Q000235:genetics*