Rewriting the tree of life?

Pandora's Box by F. S. Church. Image in the public domain. Click for source

Pandora’s Box by F. S. Church. Image in the public domain. Click for source

The Siberian permafrost has proven to be the ultimate biological time capsule. It has served as the final resting place of many stunningly preserved specimens of woolly mammoth, fuelling dreams of their eventual resurrection. It’s also the home of an underground ‘Noah’s ark’ seed bank with an archive of 100,000 seeds, protecting them in preparation for a possible apocalyptic future, thus enabling mankind to restore plant life to Earth. And tales of resurrection are not so far-fetched or à la Jurassic Park as you might think. A plant that once bloomed alongside the woolly mammoth (Silene stenophylla), has been successfully regenerated from fruit that was frozen a staggering 30,000 years ago. What’s more, last year scientists successfully revived a 30,000 year old ‘giant virus’ from deep within the frost.

Scientists emphasise that these revived viruses pose no threat whatsoever to humans (the virus actually much prefers to infect amoebae – phew, no need to panic!). So forget fears of a new plague. A far more plausible impact of such finds is that they can possibly re-write our perception of viruses – or even the tree of life itself. But how?

Viruses don’t quite fit in…

Viruses exist in a state of limbo between the realm of the living and non-living. Dubbed ‘organisms at the edge of life’, they skirt the border of what constitutes a living being – this is due to their modus operandi. Viruses are miniscule packages of genetic material surrounded by a protein shell. But they are unable to perform any of the usual biological functions of cellular organisms (for example, metabolise nutrients, excrete waste or reproduce). Viruses are pretty lifeless until they encounter a suitable target to infect – but when they do they get pretty lively.

Viruses are like parasites, hijacking cell systems in a hostile takeover for their own gain. The viruses then rely on the host cell to multiply and make new copies. The cell then becomes an unwilling Trojan horse, a treacherous cell that eventually releases virions (virus particles) which can infect the rest of the host. Devious!

Viruses break through our comfort zone with as much ease as they knock aside our body’s immune defences to infect us. There’s nothing quite like a viral outbreak to command global attention – as well as challenge our confidence in humankind’s perceived mastery of nature. We can chart the rise and fall of societies on the tiny backs of viruses -from smallpox and Spanish Flu to SARS (Severe Acute Respiratory Syndrome), Ebola and, more recently, the MERS-CoV (Middle-East Respiratory syndrome Coronavirus) outbreak in South Korea.

Physically small and genetically simple: yet when it comes to understanding them, viruses have long been placed firmly in quarantine. They even enjoy their own unique classification system. However the discovery of a new family of viruses has pretty much upended all our pre-existing ideas.

A case of mistaken identity

Mimivirus particle, electron microscopy, licensed under CC Generic Attribution 2.5. Published in PLoS Pathog. 2008 Jun 13;4(6) - click image for source

Mimivirus particle, electron microscopy, licensed under CC Generic Attribution 2.5. Published in PLoS Pathog. 2008 Jun 13;4(6) – click image for source

This virus was eventually christened ‘Mimivirus*’, short for ‘mimicking microbe virus’, because it was so big that researchers first mistook it as a bacterium. This case of mistaken identity went so far that ‘Mimivirus’ even enjoyed a brief stint under a bacterial name. But Mimivirus was in fact was the first giant virus ever found, happily thriving in amoebae.

Giant viruses are unique because of their immense size and abundance of genes. To understand the true magnitude of how massive this virus is, HIV (Human Immunodeficiency Virus) is 120 nanometres across and has just nine genes.  Mimivirus is a monster, at 600 nanometres in diameter with a staggering 979 genes. But Mimivirus was not alone. In 2011, the fittingly named Megavirus (1,120 genes) was found in seawater off the coast of Chile.  In 2013 a new family of giant viruses smashed all previous records – the Pandoraviruses.

The virus with all the gifts

In Greek mythology, the Gods created the first human woman, irresistible to both man and God. After sculpting her out of clay, the Gods then each gave her a gift.  Aphrodite gave her grace, Poseidon promised she would never drown, Hermes gave her the gift of speech, and Hera gave curiosity. Lastly, before life was breathed into her, this clay woman was given a name – Pandora, the ‘all-gifted’.

This was the namesake of the newest family of giant viruses. The Pandoraviruses were named because of the irresistible scientific contributions they possibly hold.  It also hints at the unknown, and that’s the most exciting part. Pandoraviruses weren’t just found in off the Chilean Coast – they have also appeared in contact lenses and in a Melbourne pond. Pandoraviruses are behemoths – twice as big as the Megavirus. But exactly how big?

They are so big that you only need a light microscope to see them. That’s a standard laboratory microscope. The tobacco mosaic virus was first observed the late 1800s with a light microscope, but it wasn’t until 1931 that we were able to see what viruses looked like, with the invention of the electron microscope that was 5000 times more powerful than the light microscope. The elusive viruses were revealed in all their geometric glory.  Since then entire branches of science have been developed and dedicated to the cause of catching a glimpse of a virus’ movement and appearance. How well a virus can be imaged is now a common benchmark for most microscope technologies.

The Pandoravirus’ genetic information hints at the possible impact on what we currently know about life. Only 7% of its genes match any gene known to science. Professor Claverie (one of the Pandoravirus discoverers) states that the unique genetic nature of the Pandoraviruses will blur the boundary between what is considered a living cell and a virus: “the lack of similarity…might be an indication that [Pandoraviruses] originated from a totally different primitive cellular lineage.” What’s known so far is that giant viruses are diverse and hardy (resurrection from the Siberian permafrost is no small feat, after all!) – and that species of giant virus discovered so far only infect amoebae. However, these viruses may be more complex than originally thought, and it is also possible that different types of viruses may have evolved separately – so they might not have a common ancestor.

The tree of life is one of the most evocative images of the scientific world, showing the evolutionary relationship between each organism in the world. In the classical tree of life there are three branches – Eukaryotes (plants, animals and humans) Bacteria and single-celled Archaea. Could Pandoraviruses herald the inclusion of a fourth domain?  The tree of life may need to be revised to account for these new viruses – it’s still early days, but it is worth noting that Archaea are a relatively recent addition to the tree, added when Norman Pace wrote a compelling case for their inclusion.

Science inspires a sense of wonder as new discoveries cascade down into current understanding. Once the genes of the giant viruses are decoded they may provide a new insight into the origin of viruses – as well as roles within the story of the evolution of life. Perhaps in the future a new branch will be grafted onto the tree of life.

– IvyS


She’s opened Pandora’s box…

Ivy is a PhD student investigating how HIV infects immune cells at the Kirby Institute in Sydney. Follow her on Twitter! @ivyhish

* Cute? Just me? – S.

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