Extreme variation in the prevalence of inherited male-killing microorganisms between three populations of Harmonia axyridis (Coleoptera : Coccinellidae)

Females from three populations of Harmonia axyridis (Pallas) were assayed f or two phenotypical indicators of the presence of male-killing endosymbiont s: low egg hatch-rates and strongly female-biased progenic sex ratios. Samp les from Sapporo City, Japan, and the Altai Mountains, Mongolia, but not fr om Novosibirsk, Russia, were found to contain some females displaying both of these traits. Furthermore, there was a profound difference in the preval ence of infection between the Japanese and Mongolian populations. The propo rtion of females infected from the Japanese sample was approximately 0.49, whereas that from Altai was only 0.02. The trait was inherited by more than 99% of the progeny of infected females. The trait was inherited maternally with the same high efficiency over five generations. Treatment with antibi otics produced a small increase in the production of males from the male-ki lling lines, suggesting that the male-killer is bacterial in nature. Causes of variation in prevalence are discussed, and the consequences of high lev els of infection with an efficiently transmitted male-killing microorganism on host population demography are considered.