On the mod resc model and the evolution of Wolbachia compatibility types

Cytoplasmic incompatibility (CI) is induced by the endocellular bacterium Wolbachia. It results in an embryonic mortality occurring when infected males mate with uninfected females. The mechanism involved is currently unknown, but the mod resc model allows interpretation of all observations made so far. It postulates the existence of two bacterial functions: modification (mod) and rescue (resc). The mod function acts in the males' germline, before Wolbachia are shed from maturing sperm. If sperm is affected by mod, zygote development will fail unless resc is expressed in the egg. Interestingly, CI is also observed in crosses between infected males and infected females when the two partners bear different Wolbachia strains, demonstrating that mod and resc interact in a specific manner: Two Wolbachia strains are compatible with each other only if they harbor the same compatibility type. Here we focus on the evolutionary process involved in the emergence of new compatibility types from ancestral ones. We argue that new compatibility types are likely to evolve under a wider range of conditions than previously thought, through a two-step process. First, new mod variants can arise by mutation and spread by drift. This is possible because mod is expressed in males and Wolbachia is transmitted by females. Second, once such a mod variant achieves a certain frequency, it can create the conditions for the deterministic invasion of a new resc variant, allowing the invasion of a new mod resc pair. Furthermore, we show that a stable polymorphism might be maintained in natural populations, allowing the long-term existence of "suicidal" Wolbachia strains.