Maternally inherited sex ratio distortion as a result of a male-killing agent in Spilostethus hospes (Hemiptera: Lygaeidae)

Abstract

A family composed solely of daughters was produced by a pair of milkweed bugs, Spilostethus hospes (Hemiptera: Lygaeidae). The sex ratio among the offspring of the daughters was significantly female-biased, indicating 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 was half that observed in pairs with unbiased offspring sex ratio, and pairs expressing sex ratio distortion converted to unbiased offspring sex ratio following tetracycline treatment. Successful selection for a highly female-biased sex ratio suggests that there is resistance to sex ratio distortion. Vertical transmission was incomplete and considerably reduced among females that underwent a forced delay in reproduction at cool temperatures analogous to an overwintering phase of the life cycle. An attempt to transfer bacteria horizontally by forcing early instars from a nonhost line to cannibalize host eggs was unsuccessful. With incomplete vertical transmission and no horizontal transmission, the bacterium presumably promotes its existence by boosting the reproductive success of host females, a possibility which remains to be investigated.