Male-killing Wolbachia in a flour beetle

The bacteria in the genus Wolbachia are cytoplasmically inherited symbionts of arthropods. Infection often causes profound changes in host reproductio n, enhancing bacterial transmission and spread in a population. The reprodu ctive alterations known to result from Wolbachia infection include cytoplas mic incompatibility (CI), parthenogenesis, feminization of genetic males, f ecundity enhancement, male killing and, perhaps lethality. Here, we report male killing in a third insect, the black flour beetle Tribolium madens, ba sed on highly female-biased sex ratios of progeny from females infected wit h Wolbachia. The bias is cytoplasmic in nature as shown by repeated backcro ssing of infected females with males of a naturally uninfected strain. Infe ction also lowers the egg hatch rates significantly to approximately half o f thus; observed for uninfected females. Treatment of the host with antibio tics eliminated infection; reverted the sex ratio to unbiased levels and in creased the percentage hatch. Typically Wolbachia infection is transmitted from mother to progeny, regardless of the sex of the progeny; however, infe cted T. madens males are never found. Virgin females are sterile, suggestin g that the sex-ratio distortion in T. madens results from embryonic male ki lling rather than parthenogenesis. Based on DNA sequence data, the male-kil ling strain of Wolbachia in T. madens was indistinguishable from the CI-ind ucing Wolbachia in Tribolium confusum, a closely related beetle. Our findin gs suggest that host-symbiont interaction effects may play an important rol e in the induction of Wolbachia reproductive phenotypes.