Award: OCE-1332782

Viruses are one of the most abundant entities in the ocean. Most of the viruses in the ocean infect bacteria, cyanobacteria, and eukaryotic phytoplankton. These cells form the base of the marine food web, providing energy for the animals that eat them. A significant proportion of bacterial and algal communities can be killed by viruses on a daily basis, thus viruses regulate the abundance and diversity of their hosts, and also play a key role in nutrient cycling in the oceans. Most marine viruses have a specific host range, that is they can only infect certain types of cells, yet the long-term dynamics that allow both viruses and their host cells to persist in the ocean are not well understood. In this study, we focused on viruses that infect marine cyanobacteria, abundant small photosynthetic cells. The goal of this project was to examine the genetics of virus-host interactions in order to gain a better understanding of which viruses infect which cyanobacteria in the oceans and the evolutionary consequences of these interactions. First, we used whole genome sequencing of natural cyanophage isolates collected from the coastal waters of Southern New England to examine the ecology, genomic structure, and diversification of viral communities. We found that cyanophages formed distinct clusters that remained genetically stable for a decade or longer. In other words, we were able to isolate viruses that were almost genetically identical up to 10 years apart. Nevertheless, among isolates belonging to the same cluster, highly variable regions within the genome were identified. We hypothesized that these regions contained genes that are important in helping the virus to recognize and infect particular cell types. To test this hypothesis, we then used long-term laboratory coevolution experiments to help identify the cyanophage genes that changed during interactions with their cyanobacterial hosts. In particular, we were interested in which host genes enabled a cell to become resistant to viruses and conversely which viral genes evolved to allow the virus to overcome that resistance. At the end of the 6-month co-evolution experiments, we sequenced the full genomes of over 20 Synechococcus isolates and 30 cyanophage isolates. In the Synechococcus cells, viral resistance was typically conferred by only a few single point mutations and we did not observe a correlation between the number of mutations in a Synechococcus cell and its level of resistance. In contrast, as viruses evolved to overcome the cells? resistance, we observed an accumulation of mutations over time that was directly correlated with the virus?s ability to overcome host resistance. Understanding how coevolution shapes cyanophage genomes will provide insight into the dynamics that allow both host and viral populations to coexist and persist in the marine environment. At Roger Williams University, nine undergraduates directly participated in this project during the summer and academic year. These students learned a variety of laboratory techniques as well as how to design experiments and analyze experimental data as they completed individual projects. They also gained experience in bioinformatics and scientific communication. Most students who participated in this research continued on to graduate school or obtained jobs as research assistants in academic or industry labs. In addition, inquiry-based laboratory exercises related to this research were incorporated into RWU?s BIO370L Virology lab and BIO200L Genetics lab courses. This project resulted in three research publications and several more are in preparation. All genomic data that was generated has been deposited in GenBank and is publicly available. In addition, an overview of the datasets generated in this project can be found at http://www.bco-dmo.org/project/529066. Last Modified: 11/26/2017 Submitted by: Marcia F Marston