|Ph.D Student||Daniel Schwartz|
|Subject||Genetic Hurdles Limit the Arms Race between|
Prochlococcus and the T7-like Podoviruses
|Department||Department of Biology||Supervisor||Professor Lindell Debbie|
|Full Thesis text|
Diverse populations of bacteria and their viral parasites (phages) coexist in the environment. This antagonistic coexistence is often explained through coevolutionary models, such as the arms race or density-dependent fluctuating selection, which differ in their assumptions regarding the emergence of phage mutants that overcome host resistance. Previously, phage-resistance in the abundant marine cyanobacterium, Prochlorococcus, was found to occur frequently. However, little was known about the ability of phages to overcome this resistance. This study aimed to elucidate the mechanisms enabling host-specific T7-like cyanophages to coexist with their Prochlorococcus hosts. To this end I attempted to isolate resistance-breaking mutants of several T7-like cyanophage types that infect previously selected phage-resistant Prochlorococcus strains. In some cases resistance-breaking mutants were found. These mutants had somewhat expanded host-ranges, altered fitness and, as implicated by homology analysis, mutations in tail-related genes. Comparative genomics and exploration of viral metagenomes revealed resistance-breaking genes to be among the most diverse in isolates and natural populations of T7-like cyanophages, suggesting that continuous phage-host coevolution enhances phage diversity. Intriguingly, most interactions did not yield resistance-breaking phages. As such, resistance mutations in the host raise genetic barriers to continuous arms race cycles and are indicative of an inherent asymmetry in coevolutionary capacity, with hosts having the advantage. Nevertheless, phages coexist with hosts, which I propose relies on the combined, parallel action of a limited arms race, fluctuating selection and passive host-switching within diverse communities. Together, these processes generate a dynamic network of interactions, enabling stable coexistence between hosts and phages in nature.