Frank 9627259 Most organisms have a community of bacteria, viruses and other microorganisms living within their bodies. These microorganisms are called symbionts. Some of these symbionts are parasitic, causing harm to their hosts. In other cases the hosts depend on their symbionts for nutrients, digestive enzymes and defense against other parasites. This study analyzes the evolution of symbiont life history traits. These traits include reproductive rate within the host, the time-course (life-span) of infection, mode of transmission between hosts, and harm or benefit to the host. The host's potential to control their symbionts will also be analyzed. For example, some hosts appear to manipulate the timing and mode of their symbionts' reproduction and transmission. Such host control may have a strong influence on the tendency of symbionts to evolve from harmful to beneficial partners. This research develops mathematical and computer models to clarify the basic processes that influence host-symbiont evolution. The general models are then applied to a wide range of particular systems. One example focuses on the varying degree of harm (virulence) that bacteria cause their hosts. A second example is the relationship between mode of transmission and the life history of symbionts that aid the digestive processes of their insect hosts.
