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 which to be transmitted, and maximization of the rate of transmission. This conflict is thought to be lessened when the symbiont is vertically transmitted, for here transmission depends on the maintenance of a healthy host. Past studies have confirmed this tenet, and have shown vertical transmission to select for decreased virulence. We here investigate the effects of the maternally inherited male-killing Rickettsia symbiont 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, infected females showing both decreased fecundity and longevity in comparison 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.