Scientists resurrect 700-year-old viruses

October 28, 2014 by Marcia Malory, report
An ice core containing ancient caribou feces. For thousands of years, caribou gathered on ice patches to escape summer heat and insects. The caribou feces, which contain caribou DNA, digested plants, and viruses, were frozen within layers of ice, enabling researchers to detect the genomes of ancient viruses. Credit: Brian Moorman

( —Eric Delwart of the Blood Systems Research Institute in San Francisco and colleagues have found two 700-year-old viral sequences in frozen caribou dung in an arctic ice patch. The researchers isolated part of a viral RNA genome and the complete genome of a DNA virus. They infected living plants with the DNA virus. The study appears in the Proceedings of the National Academy of Sciences.

Scientists don't know much about how viruses evolve. Understanding the structure of would increase knowledge of . However, scientists have sequenced only a small number of ancient viruses. Reconstructing ancient viruses is difficult because they change very rapidly, making it hard to see how new sequences are related to one another. In addition, the nucleic acid content of ancient viruses can degrade quickly.

In a quest to find well preserved ancient viruses, Delwart and his team analyzed layers of caribou feces in a 4,000-year-old ice patch in Canada's Selwyn Mountains. When examining nucleic acids in frozen fecal pellets extracted from a 700-year-old ice layer, they identified two sets of well preserved viral sequences.

One of these was part of the genome of an RNA , which the researchers identified as belonging to the insect-infecting genus Cripavirus. They think caribou may have ingested insects infected with the virus. Insects attracted to the caribou or the feces may also have deposited the virus on the feces and the surrounding snow.

Delwart's team was able to reconstruct the entire genome of a DNA virus from the other viral sequence. This virus did not closely resemble any modern sequenced virus. However, the team discovered distant relationships with a group of plant-infecting viruses called geminiviruses and with gemycircularviruses, found in dragonflies, fungi and animal feces.

Upon the discovery of a viral DNA and RNA in ancient caribou feces, researchers reconstituted the viral DNA genome to test whether it might infect plants. Nicotiana benthamiana plants inoculated with the ancient viral DNA displayed evidence of infection including replication of viral DNA in inoculated leaves (orange arrow) and newly emerging leaves (white arrow). Credit: Li-Fang Chen

To learn more about the DNA virus, the researchers introduced it to the plant Nicotiana benthamiana, which scientists often use as a model when studying the infectivity of cloned geminiviruses. The virus replicated itself in inoculated as well as newly emerging leaves, evidence of infection. However, the infected plants did not develop any disease symptoms. The researchers suggest this could be because Nicotiana benthamiana is not the ideal host for this virus.

Delwart's team believes that ingested the DNA virus when eating plants. Earlier studies have shown that viruses can remain infectious after passing through the digestive tracts of animals that have eaten virus-infected plants, insects or animals.

The researchers believe that as climate change speeds up the melting of arctic ice, more viral particles, which might remain infectious, could escape into the environment.

Subarctic ice patch in the Selwyn Mountains in the Northwest Territories of Canada. Alpine ice patches are unique repositories of archaeological artifacts and biological specimens that accumulate in ice layers deposited over thousands of years. Credit: Thomas D. Andrews

Explore further: Plants can 'switch off' virus DNA

More information: Preservation of viral genomes in 700-y-old caribou feces from a subarctic ice patch, PNAS, Terry Fei Fan Ng, DOI: 10.1073/pnas.1410429111

Viruses preserved in ancient materials provide snapshots of past viral diversity and a means to trace viral evolution through time. Here, we use a metagenomics approach to identify filterable and nuclease-resistant nucleic acids preserved in 700-y-old caribou feces frozen in a permanent ice patch. We were able to recover and characterize two viruses in replicated experiments performed in two different laboratories: a small circular DNA viral genome (ancient caribou feces associated virus, or aCFV) and a partial RNA viral genome (Ancient Northwest Territories cripavirus, or aNCV). Phylogenetic analysis identifies aCFV as distantly related to the plant-infecting geminiviruses and the fungi-infecting Sclerotinia sclerotiorum hypovirulence-associated DNA virus 1 and aNCV as within the insect-infecting Cripavirus genus. We hypothesize that these viruses originate from plant material ingested by caribou or from flying insects and that their preservation can be attributed to protection within viral capsids maintained at cold temperatures. To investigate the tropism of aCFV, we used the geminiviral reverse genetic system and introduced a multimeric clone into the laboratory model plant Nicotiana benthamiana. Evidence for infectivity came from the detection of viral DNA in newly emerged leaves and the precise excision of the viral genome from the multimeric clones in inoculated leaves. Our findings indicate that viral genomes may in some circumstances be protected from degradation for centuries.

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1 / 5 (6) Oct 28, 2014
Oh joy! Next off they'll try to resurrect 700 year old viruses from bodies that died of bubonic plague in the 14th century. What could possibly go wrong?

Ain't science grand?
5 / 5 (6) Oct 28, 2014
You do know that bubonic plague is a bacteria not a virus? Or that it is still infecting people is some ecountries?

Aint education grand?
5 / 5 (3) Oct 28, 2014
if they're really lucky they'll find anothe ohtze man in some of that ice.
that would be grand indeed.
5 / 5 (6) Oct 28, 2014
Oh joy! Next off they'll try to resurrect 700 year old viruses from bodies that died of bubonic plague in the 14th century. What could possibly go wrong?

The arctic is melting whether scientists study it or not. The viable viruses will be resurrected whether scientists study them or not. Wouldn't it be cool if some group of people, say scientists, were to study the viruses in a lab before they were released in the wild? Or should we just cross our fingers and hope nothing bad melts?

Yes, science is grand.
4 / 5 (4) Oct 29, 2014
I'm not concerned in the slightest, these viruii appear primarily plant based &unlikely to manipulate cells in animals to replicate, well at least, an attempt will attract attention, our immune system will likely dispatch them pretty quickly - if they survive long enough to get to that stage after all the hurdles...

In any case they may be primarily phages as its not clear how much material survived (fully intact) for exact classification.

It should be noted oceans have a tremendous number of virions, some 10^8 per cc in some regions & has been used as a poor mans antibiotic in ex USSR

Aint education grand?

Molecular genetics & variations are a fascinating & massively complex area of study, just goes to show (from computing & genetic algorithms) you don't need a singular act of creation, all u need are discontinuous materials, time & space & complexity "naturally" arises all encompassed by the imperative & brutality of nature's "Eat & be Eaten"

Ain't science grand?
4 / 5 (4) Oct 29, 2014
Give him a break, guys; he saw a movie once...
5 / 5 (1) Oct 29, 2014
Let's panic before we know what it does!

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