Originally published as Genetics Published Articles Ahead of Print on April 16, 2005.

Genetics, Vol. 170, 495-507, June 2005, Copyright © 2005
doi:10.1534/genetics.103.015198

Exploring the Evolution of Wolbachia Compatibility Types

A Simulation Approach

Sylvain Charlat^*,1, Claire Calmet, Olivier Andrieu^* and Hervé Merçot^*

^* Institut Jacques Monod, Laboratoire Dynamique du Génome et Evolution, 75005 Paris, France
Museum National d'Histoire Naturelle, Service de Systématique Moléculaire, 75005 Paris, France

¹ Corresponding author: Biology Department, University College London, 4 Stephenson Way, London, NW1 2HE, United Kingdom.
E-mail: s.charlat{at}ucl.ac.uk

Wolbachia-induced cytoplasmic incompatibility (CI) is observed when males bearing the bacterium mate with uninfected females or with females bearing a different Wolbachia variant; in such crosses, paternal chromosomes are lost at the first embryonic mitosis, most often resulting in developmental arrest. The molecular basis of CI is currently unknown, but it is useful to distinguish conceptually the male and female sides of this phenomenon: in males, Wolbachia must do something, before it is shed from maturing sperm, that will disrupt paternal chromosomes functionality [this is usually termed "the modification (mod) function"]; in females, Wolbachia must somehow restore embryonic viability, through what is usually called "the rescue (resc) function." The occurrence of CI in crosses between males and females bearing different Wolbachia variants demonstrates that the mod and resc functions interact in a specific manner: different mod resc pairs make different compatibility types. We are interested in the evolutionary process allowing the diversification of compatibility types. In an earlier model, based on the main assumption that the mod and resc functions can mutate independently, we have shown that compatibility types can evolve through a two-step process, the first involving drift on mod variations and the second involving selection on resc variations. This previous study has highlighted the need for simulation-based models that would include the effects of nondeterministic evolutionary forces. This study is based on a simulation program fulfilling this condition, allowing us to follow the evolution of compatibility types under mutation, drift, and selection. Most importantly, simulations suggest that in the frame of our model, the evolution of compatibility types is likely to be a gradual process, with new compatibility types remaining partially compatible with ancestral ones.

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