Vertical Transmission and Evolution of Mutualism from Parasitism

Using an evolutionarily stable strategy (ESS) model, it is shown that vertical transmission, defined as the direct transfer of infection from a parent host to its progeny, is an important factor which can urge reduction of parasite virulence. Evolution of the vertical transmission rate from both points of view, the parasite and the host, is analyzed. There is a critical level on the rate, below which an evolutionary conflict arises (the parasite would want to increase the rate while the host would not), and above which both sides would correspond to increase the rate. Therefore, once the parasite dominates the evolutionary race so as to overcome the critical level, the one-way evolution begins toward a highly mutualistic relationship with a high vertical transmission rate. The changes in other parameters may decrease the critical level, initiating the one-way evolution. However, changes in traits, probably developed through long interrelationship in parasitism, do not necessarily induce the evolution of mutualism. Establishment of the ability to make use of metabolic and digestive wastes from the partner certainly facilitates the evolution of mutualism, while improvements in reproductive efficiency of the parasites and reduction of negative effects from exploitation in hosts contrarily disturb mutualism. Finally, available data are discussed in the light of predictions of the model.