The sympatric lifestyle of giant viruses: contact tracing and fitness through mobile genetic elements

Giant viruses appear to be ubiquitous in soil and aquatic environments, infecting a wide range of protist hosts. As lytic viruses, they are important regulators in nutrient and energy cycles and key influencers of microbial community composition. The recent discovery of giant viruses challenged previous assumptions and blurred the sharp division between viruses and cellular life. Besides large particle sizes, giant viruses possess complex ""chimeric"" genomes, including genes that were likely acquired from their hosts and bacteria that parasitise the same hosts. Unique is the presence of prokaryotic-like mobile genetic elements (MGEs) that are speculated to aid giant viruses in defence against the host immune system or in direct competition for resources with other viruses or bacteria. Contrarily, bacteria may use MGEs to help the hosts counteract viral infections. Our current knowledge on the factors promoting giant virus diversity and maintenance of the virus-host balance in nature, are largely unknown. In the proposed project, I will investigate the role of MGEs in the evolution and ecology of giant viruses. I postulate that the presence of MGEs plays a crucial role in the competition between giant viruses and other parasites infecting the same hosts. Using co-infection experiments, as well as cutting-edge molecular, microscopy, and sequencing techniques, I will investigate viral competitive fitness as well as physical and molecular interactions between selected partners. By developing a highly specific giant virus genome editing tool, I will rigorously test whether MGEs can provide giant viruses with higher fitness. Moreover, I will combine cell sorting with metagenome analysis of two selected habitats, to unravel how MGEs are distributed in a natural ecosystem. My overarching goal is to elucidate the molecular dialogue between viruses, bacteria, and their hosts, and to use MGEs as a tool to trace the evolutionary history of this unique group of viruses.""