Widespread genomic islands are hotspots of genome variations and mosaicism in giant viruses

Giant viruses, already famous for carrying genomes larger than those of some bacteria, appear to owe much of their extraordinary variability to specialized regions of their DNA that behave like evolutionary accelerators. A new study published in Nature Communications maps these regions, known as genomic islands, across the phylum Nucleocytoviricota and concludes that they are far more than a genetic curiosity: they are pervasive hotspots of gene gain, loss, and rearrangement that may underpin the viruses' ability to adapt to their hosts and even to trade genes with bacteria.

The research, carried out by Benjamin Minch and Mohammad Moniruzzaman, set out to address a long-standing puzzle in virology. Nucleocytoviricota, the phylum that includes many of the so-called giant viruses, possess exceptionally large and mosaic genomes whose composition can differ dramatically even among close relatives. These viruses, which infect a wide range of eukaryotic hosts including algae, amoebae, and other protists, routinely defy conventional expectations of what a virus should look like. While typical viruses carry only a handful of genes, giant viruses can harbor hundreds or even more than a thousand, sometimes including genes involved in DNA repair, protein folding, and metabolic processes once thought to be exclusive to cellular organisms. Yet the mechanisms that generate and maintain this plasticity have remained poorly understood. Genomic islands, which are dynamic stretches of DNA that act as major engines of diversification and adaptation in bacteria, seemed like an obvious candidate, but their contribution to giant virus evolution had never been systematically explored.

The concept of the genomic island is well established in bacterial genetics. In prokaryotes, these are discrete regions of the chromosome, often acquired through horizontal gene transfer and frequently flanked by mobile genetic elements such as tRNAs, integrases, or repeat structures, where genes of foreign origin cluster together. Bacterial genomic islands commonly encode traits with clear ecological value: pathogenicity factors, antibiotic resistance genes, metabolic pathways for exploiting unusual nutrients, or secretion systems for interacting with other organisms. Because islands concentrate functionally important novelty in mutable corners of the genome, they allow bacterial lineages to adapt quickly without destabilizing the core genes needed for basic survival. Whether viruses, and particularly viruses with genomes as large and complex as those of Nucleocytoviricota, employ an analogous strategy has been an open question.

To fill that gap, the researchers assembled a dataset of 369 high-quality giant virus genomes. Crucially, this collection included both cultured isolates, which can be studied under laboratory conditions, and long-read metagenome-assembled genomes, which are reconstructed directly from environmental samples using sequencing technologies that produce long, continuous DNA reads. Long-read approaches are particularly valuable in this context because they reduce the fragmentation that has historically made it difficult to assemble the repetitive and structurally complex regions where genomic islands tend to reside. Short-read sequencing, which dominated the genomics era for years, generates millions of brief reads that must be stitched together computationally; repetitive sequences longer than a single read cannot be resolved unambiguously, often collapsing into truncated or scrambled assemblies. Because genomic islands are precisely the kinds of regions rich in repeats, duplications, and recently inserted foreign DNA, earlier short-read-based surveys may have systematically underestimated their presence. By combining the two types of genomes, the team was able to survey genomic islands across a broad taxonomic and ecological range of Nucleocytoviricota, spanning viruses collected from marine, freshwater, and other environments.

The scale of the finding exceeded even the authors' expectations. Across this dataset, the analysis identified 307 genomic islands distributed among more than half of the genomes examined. This prevalence demonstrates that genomic islands are not rare anomalies confined to a few unusual lineages but a widespread and recurring feature of giant virus genome architecture. Wherever the researchers looked within the phylum, they found these dynamic regions punctuating otherwise more stable stretches of viral DNA. That consistency across diverse lineages suggests that the underlying biology, whatever mechanisms create and maintain islands, is deeply embedded in the evolution of the group rather than being a peculiarity of one branch of the viral family tree.

What sets genomic islands apart from the rest of the genome is their tendency to vary. The study found that these regions are frequently associated with genomic hypervariability, meaning that their gene content and organization change rapidly compared with neighboring parts of the genome. In practical terms, two giant virus genomes that are otherwise highly similar may differ substantially within their islands, with genes appearing, disappearing, or shuffling position. The comparative analyses revealed frequent gains and losses of genes within islands, as well as rearrangements, even among genomes that were nearly identical elsewhere. This pattern marks the islands as hotspots of genome diversification, places where evolutionary change concentrates while the rest of the genome remains comparatively conserved. Such a division of labor, between a stable genomic core and a volatile accessory periphery, mirrors patterns long recognized in bacterial pangenomics, where core genes maintain essential functions while accessory genes, often in islands, mediate ecological specialization.

The functional profile of the genes inside these islands offers a clue to why the viruses maintain such volatile real estate. The researchers found that genes involved in host interaction were enriched within the islands, with surface adhesion proteins standing out as a particularly prominent category. Surface adhesion proteins are molecules that help viruses attach to their host cells, a critical step in infection. For viruses that infect eukaryotic hosts, the initial recognition of a compatible cell surface is often the decisive determinant of host range: a virus that cannot bind cannot infect, no matter how well its genome is suited to replication once inside. In the context of the ongoing arms race between viruses and their hosts, rapid diversification of attachment machinery could allow a virus to recognize new host surfaces or evade host defenses that have evolved to block familiar binding strategies. The enrichment of such genes in hypervariable regions suggests that genomic islands serve as testing grounds for host adaptation, generating variation in exactly the functions most likely to influence infection success.

Beyond host adaptation, the study uncovered evidence pointing to an unexpected source of the island gene repertoire: bacteria. Many of the genomic islands were enriched in bacterial homologs, meaning they contained genes whose closest relatives are found in bacterial genomes rather than in other viruses. More strikingly, several islands exhibited synteny, conservation of gene order and organization, with genomic regions recovered from bacterial genomes found in the same environments as the viruses. Synteny of this kind is difficult to explain by chance. Genes tend to be reshuffled over evolutionary time, so two DNA segments drawn at random from unrelated organisms would almost never preserve the same gene order. The researchers interpret the observed synteny as support for large-scale genetic exchange between bacteria and giant viruses. In other words, stretches of DNA may have moved wholesale between co-occurring bacterial and viral genomes, seeding the islands with bacterial-derived genes arranged in the same order as in their bacterial counterparts.

This proposed exchange has significant implications for how scientists think about the mosaic nature of giant virus genomes. The mosaicism of Nucleocytoviricota has long been noted, with different genes in a single genome suggesting wildly different evolutionary origins, some resembling bacterial genes, others eukaryotic, and others related to other viruses. Competing hypotheses about the origin of giant viruses, whether they descended from an ancient viral lineage that grew by accumulating genes or acquired much of their complexity by harvesting DNA from hosts and other microbes, have fueled debate for years. If genomic islands are conduits for gene flow from bacteria, they could be one of the principal mechanisms by which viruses acquire foreign DNA, incorporating it into their genomes and, over time, reshaping their metabolic and interaction capabilities. The findings thus provide a framework for understanding genome plasticity, mosaicism, and the adaptive potential of giant viruses, positioning islands as the engines driving much of this change.

The evolutionary scenario that emerges is one in which giant virus genomes are not static blueprints but dynamic mosaics, continuously renovated at designated hotspots. Genes involved in host interaction are recruited into islands, where turnover is rapid and rearrangement common, allowing viral lineages to experiment with new surface proteins and other interaction factors. Meanwhile, genetic exchange with co-occurring bacteria supplies fresh raw material, some of which may prove useful in the virus-host arms race. This combination of internal volatility and external gene acquisition helps explain both the sheer size of giant virus genomes and their bewildering diversity of gene content. It also suggests a functional logic to the architecture: by concentrating volatility in islands, giant viruses can innovate aggressively while preserving the core machinery that replication requires, an arrangement that balances stability and change.

The study's methods merit attention for how they strengthen these conclusions. By using long-read metagenome-assembled genomes alongside cultured isolates, the researchers mitigated a key limitation of earlier work, which relied heavily on short-read assemblies that can break apart or misassemble the very repetitive regions where islands concentrate. Characterizing islands across more than 50 percent of a large and diverse set of genomes also lends statistical weight to the claims of pervasiveness, while the comparative approach of examining closely related genomes allowed the team to detect gain, loss, and rearrangement events that would be invisible when comparing only distant relatives. Comparisons among close relatives function like snapshots taken at short intervals: differences that accumulate between them reveal recent evolutionary activity, whereas comparisons across deep divergence wash such signals out.

As with any study, there are caveats and boundaries to the conclusions. Metagenome-assembled genomes, even those generated with long reads, are reconstructions rather than complete, verified sequences, and some uncertainty about assembly accuracy at the finest scale can remain. The evidence for bacterial-viral gene exchange, while compelling in its synteny signal, is correlational rather than a direct observation of DNA transfer; demonstrating the mechanism and direction of exchange in the laboratory would be a logical next step. Additionally, the functional enrichment findings identify categories of genes, such as surface adhesion proteins, whose roles in host adaptation are suggested by their location and variation patterns, but experimental tests of how specific island genes affect infectivity would be needed to confirm causation.

Even with these limitations, the work reshapes the picture of giant virus evolution in a way that resonates with decades of bacterial genomics. In bacteria, genomic islands acquired through horizontal gene transfer have long been recognized as keys to ecological innovation, encoding traits like antibiotic resistance and metabolic capabilities that allow lineages to colonize new niches.