MATERNALLY INHERITED SEX-RATIO DISTORTION AS A RESULT OF A MALE-KILLING AGENT IN SPILOSTETHUS HOSPES (HEMIPTERA, LYGAEIDAE)

A family composed solely of daughters was produced by a pair of milkwe ed bugs, Spilostethus hospes (Hemiptera: Lygaeidae). The sex ratio amo ng the offspring of the daughters was significantly female-biased, ind icating that sex ratio distortion is heritable. The following results suggest that sex ratio distortion is caused by a maternally inherited, male-killing bacterium: females transmitted sex ratio distortion but males did not, egg hatch among pairs expressing sex ratio distortion w as half that observed in pairs with unbiased offspring sex ratio, and pairs expressing sex ratio distortion converted to unbiased offspring sex ratio following tetracycline Successful selection for a highly fem ale-biased sex ratio suggests that there is to sex ratio distortion. V ertical transmission was incomplete and considerably reduced among fem ales that underwent a forced delay in reproduction at cool temperature s analogous to an overwintering phase of the life cycle. An attempt to transfer bacteria horizontally by forcing early instars from a nonhos t line to cannibalize host eggs was unsuccessful. With incomplete vert ical transmission and no horizontal transmission, the bacterium presum ably promotes its existence by boosting the reproductive success of ho st females, a possibility which remains to be investigated.