Wolbachia is a bacterium that induces cytoplasmic incompatibility (CI) resulting in increased mortality of host offspring when infected males mate with females uninfected by the same Wolbachia strain. Theoretical models predict that the Wolbachia strain inducing the highest CI level overwhelms others that induce lower CI levels within a host population. However, intermediate CI levels are widespread because the hosts evolved countermeasures for moderating CI levels. If this scenario is correct, novel Wolbachia strains should cause higher CI levels than those share coevolutionary history with their host.

We tested this hypothesis by injecting novel Wolbachia strains to Callosobruchus analis infected with Wolbachia in natural conditions. Weak CI levels were observed when naturally infected host males were mated with uninfected host females cured with antibiotics. High CI levels were observed when naturally infected females were crossed with males injected with a novel Wolbachia strain. Conversely, CI levels were moderated when the injected males were mated with females that once infected with the novel Wolbachia strain but cured with antibiotics. Our results support the scenario that Wolbachia CI levels were tamed in the history of Wolbachia-host interaction.