Experimental Host-Parasite Co-Evolution in a Changing Environment

Parasites with exclusive vertical transmission from host parent to offspring are an evolutionary puzzle. Any fitness costs for infected hosts risk the selective elimination of these parasites because their fitness is linked to host reproduction. One of the main evolutionary transitions from parasitism towards beneficial or mutualistic associations may therefore encompass a change from horizontal transmission to vertical transmission. In this thesis, the experimental evolution study on Paramecium and Holospora supports this hypothesis. The parasite nearly entirely lost horizontal transmission capacity in a treatment favouring vertical transmission and low virulence. However, many vertically transmitted parasites e.g. Caedibacter taeniospiralis impose detectable costs to their hosts. This endosymbiont imposes context-dependent costs to its host Paramecium tetraurelia. Fitness of infected paramecia was reduced in resource-limited conditions at all experimentally tested temperatures (16-32°C). These universal fitness costs along the temperature gradient necessitate universal cost compensation that can be the ‘killer trait’ that eliminates uninfected competitors. At acute heat stress the loss of infection indicates that cost compensation is impossible, thereby restricting conditions for parasite persistence. Surprisingly, the parasite persists in permanent stress and optimal temperature conditions. Caedibacter was able to adapt to high temperature conditions by increasing its number in the populations but without reducing virulence in high temperature conditions. Acute and intense stress harms the parasite and causes its extinction but the parasite was able to evolve and adapt to stress conditions. Moreover, the parasite reacts exactly in the opposite direction as it was expected. They do not suffer from stressful conditions, they benefit