A correlated response of a parasite's virulence and life cycle to selection on its host's life history

We demonstrate a correlated response of the virulence and the mode of trans mission of the microsporidian parasite Edhazardia aedis to selection on the age at pupation of its host, the mosquito Aedes aegypti. We selected three lines of mosquitoes each for early or late pupation and ex-posed the larva e after zero, two and four generations of selection to a low and a high con centration of the parasite's spores. Before selection the parasites induced a similar level of mortality in the six lines; after four generations of s election mortality was higher in the mosquitoes selected for late pupation than in those selected for early pupation. Overall, parasite-induced mortal ity was positively correlated with the mean age at pupation of the matching uninfected line. When they died, mosquitoes selected for early pupation ha rboured mostly binucleate spores, which are responsible for vertical transm ission. Mosquitoes selected for late pupation were more likely to harbour u ninucleate spores, which are responsible for horizontal transmission. The p arasite enhanced this tendency for horizontal transmission by prolonging th e larval period in the lines selected for late pupation, but not in the one s selected for early pupation. These results suggest that the genetic basis of the mosquito's age at pupation helps to determine the parasite's mode o f transmission: parasites in rapidly developing mosquitoes are benign and t ransmit vertically, while parasites in slowly developing mosquitoes are vir ulent and transmit horizontally. Thus, as the host's life history evolves, the parasite's performance changes, because the host's evolution changes th e environment in which the parasite develops.