Two male-killing Wolbachia strains coexist within a population of the butterfly Acraea encedon.

Inherited bacteria that kill male hosts early in their development are known from five insect orders. We ask to what extent the incidence of male-killers might be restricted by the rate at which new host-parasite interactions arise, by testing whether multiple male-killers have invaded a single host species. In Uganda, the butterflies Acraea encedon and A. encedana are both infected by the same strain of male-killing Wolbachia and there was no evidence of variation within the population. In Tanzanian A. encedon however, two phylogenetically distinct strains of male-killing Wolbachia were found within the same population. If this pattern of male-killer polymorphism is found to be general across infected species, it suggests that new male-killing infections arise frequently on an evolutionary time scale. Whether this polymorphism is stable, and what forces may be maintaining it, are unknown.