How hidden viruses wake up inside seaweed and pass on to future generations

Researchers at the Max Planck Institute for Biology Tübingen have shown that giant viruses long thought to exist only as fleeting, free-living particles that can embed themselves permanently in the genome of a multicellular host, lie dormant for generations and then wake up on demand. Their study, published in Nature Microbiology, challenges fundamental assumptions about how giant viruses operate and establishes a powerful new model for studying viral latency in complex organisms.

Viruses that hide in plain sight

Giant viruses are among the most remarkable biological entities on Earth. Larger and more genomically complex than many bacteria, they can cause major issues in marine ecosystems, infecting algae and microbes across the world's oceans. Yet despite their abundance and ecological role, their infection strategies in multicellular hosts have remained poorly understood. In particular, whether giant viruses are capable of latency, the ability to persist silently inside a host before reactivating, has been an open question.

This new study answers that question. Working with Ectocarpus, a brown alga that serves as a model organism for multicellular biology, the team discovered that the alga's genome harbors complete, intact giant virus sequences, known as giant endogenous viral elements (GEVEs). These GEVEs are fully functional and capable of producing infectious viral particles.

"These are not genomic fossils," said Carole Duchêne, postdoc in the Department of Algal Development and Evolution. "They are active, regulated, and transmissible. The virus has effectively made itself at home inside the host genome and found a way to persist across generations."

A precisely controlled awakening

The researchers found that the dormant virus does not reactivate randomly. Instead, its awakening is tightly controlled by two distinct signals: the developmental state of the host and environmental temperature. The virus switches on specifically in reproductive cells of the seaweed, called the gametangia and sporangia, where it hijacks these structures, turning them into virus-producing factories and preventing the formation of gametes and spores. Outside these specific cells and conditions, it remains completely silent.

Using long-read genome sequencing, transcriptomics, classical genetics, and CRISPR/Cas gene editing, the team resolved the precise sites where the viral genome integrates into the host's chromosomes, elucidated the mechanism of integration, and demonstrated that the integrated element is capable of autonomous replication. Critically, by using CRISPR to delete the entire viral element from the host genome, they directly confirmed that the integrated sequence, and not some external source, is responsible for producing infectious viral particles.

Inherited like a gene, transmitted like a virus

Perhaps most strikingly, the research team showed that the viral element is stably transmitted through the host's germline from one generation to the next, behaving, in genetic terms, like a super-gene. This means the virus has two modes of transmission: vertically, passed from parent to offspring via the genome, and horizontally, through the infectious particles it produces upon reactivation. This dual strategy of latent inheritance combined with periodic reactivation and release has never before been documented for a giant virus in a multicellular eukaryote.

"The virus has evolved a remarkably sophisticated strategy," said Susana Coelho, Director of the Department of Algal Development and Evolution. "It hides inside the host, gets passed on to every offspring, and then selectively wakes up at just the right moment and in just the right cell type."

Brown algae represent the third most complex multicellular organism on the planet, having evolved complex body plans entirely independently of animals and plants. They are a foundational species in coastal marine ecosystems analogous in their ecological importance to trees in a forest. Brown algae are increasingly threatened by climate change. Understanding the viruses that infect them, and how those viruses shape host evolution and population dynamics, is therefore of great ecological significance.

More broadly, the findings challenge long-held assumptions in virology. Latency has been extensively studied in animal viruses, particularly herpesviruses and retroviruses such as HIV, but the idea that giant dsDNA viruses, a phylogenetically distinct and enormously diverse group, could employ the same strategy in a multicellular host opens entirely new avenues of research.

The Ectocarpus–Phaeovirus system now provides the scientific community with a tractable, genetically manipulable model to study the molecular mechanisms of viral latency, inheritance, and reactivation outside of traditional animal and plant frameworks.

Publication details

Carole Duchêne et al, Latent endogenous giant viruses drive active infection and inheritance in a multicellular algal host, Nature Microbiology (2026). DOI: 10.1038/s41564-026-02361-z