Male-killing Wolbachia and mitochondrial DNA: selective sweeps, hybrid introgression and parasite population dynamics

Mitochondrial DNA (mtDNA) sequences are widely used as neutral genetic markers in insects. However, patterns of mtDNA variability are confounded by the spread of maternally transmitted parasites, which are genetically linked to the mitochondria. We have investigated these effects in the butterflies Acraea encedon (which is host to two strains of male-killing Wolbachia bacteria) and A. encedana (which is host to one strain). Within a population, the mitochondria are in linkage disequilibrium with the different male-killers. Furthermore, there has been a recent selective sweep of the mtDNA, which has led to the loss of mitochondrial variation within populations and erased any geographical structure. We also found that one of the male-killers, together with the associated mtDNA, has introgressed from A. encedana into A. encedon within the last 16,000 years. Interestingly, because butterflies are female heterogametic, this will presumably have also led to the introgression of genes on the W sex chromosome. Finally, in A. encedon the mitochondria in uninfected females are unaltered by the spread of the male-killer and have diverse, geographically structured mtDNA. This means we can reject the hypothesis that the male-killer is at a stable equilibrium maintained by imperfect transmission of the bacterium. Instead, some other form of balancing selection may be maintaining uninfected females in the population and preventing the species from going extinct due to a shortage of males.