Abstract
Complex virus-virus interactions can arise when multiple viruses coinfect the same host, impacting infection outcomes with broader ecological and evolutionary significance for viruses and host. Yet, our knowledge regarding virus competition is still limited, especially for single-celled eukaryotic host-virus systems. Here, we report on mutual interference of two dsDNA viruses, MpoV-45T and MpoV-46T, competing for their Arctic algal host Micromonas polaris. Both viruses affected each other's gene expression and displayed reduced genome replication during coinfection. MpoV-45T was the dominant virus, likely due to interference in the DNA replication of is competitor. Even when its coinfection was delayed, the dominant virus still prevailed while genome production of the other virus was strongly suppressed. This contrasts with typical superinfection exclusion, where the primary infection prevents secondary infection by other viruses. Higher temperature made the suppressed virus a stronger competitor, signifying that global warming is likely to alter virus-virus interactions in Arctic waters.