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Viruses conventionally encode their genetic material in either DNA or RNA, but not both. In April 2012, researchers at Portland State University identified a rare virus whose genome combines these two nucleic acids, raising the question of whether this is a one‑off event or part of a broader, unexplored viral diversity.
While virtually all cellular life stores hereditary information in DNA, viruses present a fascinating exception. Some viral genomes are DNA‑based, while others, such as HIV and influenza, are RNA‑based. Both molecules share a backbone of phosphodiester bonds, yet RNA contains the base uracil—absent in DNA—and an additional oxygen atom on its ribose sugar, rendering it chemically less stable and more prone to degradation.
Longstanding virology principles held that a viral genome is either DNA or RNA, not a mix. The discovery of the RNA‑DNA hybrid virus (RDHV) from Boiling Springs Lake in Lassen Volcanic National Park challenged that view. The RDHV’s genome is predominantly DNA, yet one of its genes closely mirrors sequences found only in RNA viruses, suggesting that at some point the virus incorporated an RNA‑derived element into its DNA backbone.
The virus was isolated by filtering lake water and subjecting the captured particles to DNA sequencing. Consequently, its host range, ecological role, abundance, and environmental stability remain unknown. To date, RDHV is the sole documented case of a virus with a hybrid DNA‑RNA genome.
Two plausible mechanisms could explain how an RNA gene entered a DNA viral genome. First, co‑infection of a single host cell by an RNA virus and a DNA virus could have allowed reverse transcription of an RNA segment into DNA, which then became incorporated into the viral genome. Second, a direct ligation event might have joined an RNA strand to a DNA strand within a co‑infected cell. Current data cannot conclusively favor one scenario over the other, nor determine whether such hybrids confer any selective advantage.
The oceans harbor an estimated millions of viruses per milliliter of seawater, most of which target bacteria. Despite extensive sequencing efforts, the vast majority of oceanic viruses remain unnamed and poorly understood. Researchers scanned Global Ocean Survey datasets for sequences resembling RDHV and identified several matches, all belonging to as‑yet unclassified viruses. This intriguing signal suggests that additional RNA‑DNA hybrid viruses may exist in marine environments, though targeted searches are needed to confirm their presence.
