THE EFFECT OF INFECTION WITH MALE-KILLING RICKETTSIA ON THE DEMOGRAPHY OF FEMALE ADALIA-BIPUNCTATA L. (2-SPOT LADYBIRD)

Virulence of symbionts is considered to be the evolutionary product of conflicting selection pressures: maintenance of a live host from whic h to be transmitted, and maximization of the rate of transmission. Thi s conflict is thought to be lessened when the symbiont is vertically t ransmitted, for here transmission depends on the maintenance of a heal thy host. Past studies have confirmed this tenet, and have shown verti cal transmission to select for decreased virulence. We here investigat e the effects of the maternally inherited male-killing Rickettsia symb iont of Adalia bipunctata, the two spot ladybird, on the demography of the female host. Although no effect of infection on larval demography was observed, infection detrimentally affected the adult host, infect ed females showing both decreased fecundity and longevity in compariso n to uninfected controls. These findings are discussed in relation to both the dynamics of the male-killing Rickettsia and the factors which affect the level of virulence of symbionts. Three hypotheses are put forward to explain why inherited symbionts are not avirulent in their female host.