Sex-ratio-distorting Wolbachia causes sex-role reversal in its butterfly host

Sex-role-reversed mating systems in which females compete for males and mal es may be choosy are usually associated with males investing more than fema les in offspring. We report that sex-role reversal may also be caused by se lfish genetic elements which distort the sex ratio towards females. Some po pulations of the butterflies Acraea encedon and Acraea encedana are extreme ly female biased because over 90% of females are infected with a Wolbachia bacterium that is maternally inherited and kills male embryos. Many females in these populations are virgins suggesting that their reproductive succes s may be limited by access to males. These females form lekking swarms at l andmarks in which females exhibit behaviours which we interpret as function ing to solicit matings from males. The hypothesis that female A. encedon sw arm in order to mate is supported by the finding that, in release-recapture experiments, mated females tend to leave the swarm while unmated females r emained. This behaviour is a sex-role-reversed form of a common mating syst em in insects in which males form lekking swarms at landmarks and compete f or females. Female lekking swarms are absent from less female-biased popula tions and here the butterflies are instead associated with resources in the form of the larval food plant.