Rapid accumulation of a vertically transmitted parasite triggered by relaxation of natural selection among hosts

The fate of a parasite with exclusively vertical transmission depends critically on its virulence, the magnitude of its negative impact on fitness of the infected host. Relaxation of natural selection within the host population should favour the parasite by reducing this impact. We studied the dynamics of the sigma virus, a vertically transmitted parasite of Drosophila melanogaster, within a captive host population kept under relaxed natural selection. The frequency of infected flies in the population grew from 5% to 70% in just 15 generations. In contrast, strong natural selection among the host curbed the propagation of the virus. Two of 15 populations originating from a pair of infected parents and kept under intense competition for 10 generations have lost the virus completely. The implications of these findings for host– parasite co-evolution are discussed.