Spatial patterns of coevolution in multispecies host-parasite interactions

Infectious diseases such as Malaria and Swine Flu continue to cause major problems to human health while viral diseases, such as Foot and Mouth, and bacterial pathogens, like Leaf Blight, are a major threat to our food supply. Similarly, infectious diseases are widespread in nature and pose a severe threat to our environment. Since these diseases cause significant harm to their plant or animal hosts they, like all other organisms, evolve through time. The ability of bacterial pathogens to evolve resistance to antibiotics is of increasing concern and has fuelled a continued search for alternative ways to control the spread of disease. One promising alternative is the use of bacteriophages (viruses that infect and burst open bacterial cells) to control the growth of bacterial populations. 'Phage therapy', in which combinations of bacteriophage viruses are used to target and infect specific pathogenic strains of bacteria, is beginning to emerge as a safe and commercially viable management strategy. Of course, bacterial hosts can also evolve to defend themselves against parasites, such as phage, and it is thus crucial to study the 'coevolution' between phage and their bacterial hosts. My proposal will examine a number of key issues central to understanding host-parasite coevolution. First, I will determine whether phage are coevolving with the bacterial pathogen, Pseudomonas syringae within leaves of tomato plants (where the bacteria is an important agricultural pathogen) and within the xylem or phloem of horse chestnut trees (where the bacteria is causing bleeding canker disease and threatening natural populations). I am using these two systems so that I can compare the interaction between bacteria and phage in nature with results from controlled, laboratory experiments, allowing me to directly investigate the process and implications of coevolution. I will examine both how the plant environment influences the coevolution between bacteria and phage and how infection by phage influences the amount of damage that the bacteria cause to their plant host. This work will be critical to evaluating the long-term implications of phage therapy as a mechanism for controlling bacterial disease. Further, this research will emphasise that coevolution is often more complex than simple, two species interactions and that understanding multi-species interactions is key to making predictions of how diseases will evolve in nature.