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Evolution of incompatibility-inducing microbes in subdivided host populations

Ralph Haygood1, Michael Turelli

  • 1Department of Evolution and Ecology and Center for Population Biology, University of California, Davis, California 95616, USA. rhaygood@duke.edu

Evolution; International Journal of Organic Evolution
|January 22, 2009
PubMed

Insights

Cytoplasmic incompatibility (CI) bacteria spread via reduced egg hatch. In subdivided populations, effective fecundity, not CI intensity, drives bacterial strain selection, though demographic fluctuations can weakly favor stronger CI.

Area of Science:

  • Microbiology
  • Evolutionary Biology
  • Population Genetics

Background:

  • Maternally inherited bacteria induce cytoplasmic incompatibility (CI) in insects, arthropods, and nematodes, reducing egg hatch when infected males mate with uninfected females.
  • CI is a key mechanism driving the spread of these endosymbionts within host populations.
  • In panmictic (unstructured) populations, selection favors bacterial strains with higher effective fecundity (infected progeny per infected female), rather than directly on CI intensity.

Purpose of the Study:

  • To investigate the evolutionary dynamics of CI-inducing bacteria in subdivided host populations.
  • To determine how host population structure influences selection pressures on bacterial strains with varying CI intensity and effective fecundity.

Main Methods:

  • Analysis using deterministic and stochastic population models.
  • Modeling considers host population subdivision, local density regulation (soft selection), and demographic fluctuations.
  • Examined the impact of varying infection frequencies and initial strain frequencies across population patches.

Main Results:

  • Effective fecundity remains the primary target of selection, even in subdivided populations.
  • Under soft selection, variations in infection frequencies can alter strain frequencies, favoring stronger CI only under specific initial conditions.
  • Demographic fluctuations can weakly favor stronger CI by maintaining frequency variation, but this effect is often outweighed by small increases in effective fecundity.

Conclusions:

  • Host population subdivision does not fundamentally alter selection favoring effective fecundity over CI intensity.
  • While demographic fluctuations can slightly promote stronger CI, selection for increased effective fecundity is a more potent evolutionary force.
  • Explaining the widespread prevalence of CI may necessitate considering selection pressures beyond those within a single host species, such as interspecies host transfer.

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