Findings that reshape our understanding of giant virus evolution
The researchers behind this study reconstructed 1,202 mirusvirus genomes from thousands of marine and freshwater samples. Characterizing these genomes formally establishes mirusviruses as a second phylum of giant viruses, alongside the nucleocytoviruses.
Among the major findings of this study:
✅ Giant genomes organized into complex modules, with a functional repertoire similar to that of known giant viruses but with a unique genetic architecture.
✅ An abundance of spliceosomal introns, several hundred per genome, inserted into key structural genes (capsid, tails, portals). These introns often contain endonucleases, suggesting that they are active, mobile, and capable of spreading within the viral genome.
✅ Mirusviruses lack the cytoplasmic replication machinery typical of other giant viruses. This indicates a direct dependence on the host cell nucleus for replication, transcription, and splicing.
✅ From an ecological perspective, they are ubiquitous, especially in surface ocean waters, and infect protists that are key to the functioning of marine ecosystems.
Phylogenetic analyses show that mirusviruses evolved to replicate in the cytoplasm in some lineages and in the nucleus in others. It is highly likely that mirusviruses conquered the nuclear niche over the course of evolution. This shift may explain their massive diversification and their current ecological success.
A new face of marine virology
By revealing giant viruses “with an eukaryotic flair”, rich in introns and residing in the nuclei of their hosts, this study pushes the boundaries of our knowledge in virology. It opens the way to key research avenues:
into viral splicing mechanisms, until now extremely rare among viruses,
into the influence of mirusviruses on the genomic evolution of marine eukaryotes,
and into their role in the dynamics of ocean ecosystems.
Mirusviruses now stand as a major new component of known marine biodiversity and a crucial missing link for understanding the evolutionary history of giant viruses.
Contact: Tom Delmont