Long mistaken for bacteria, giant viruses (as their name suggests) are massive. But these viruses are even weirder than we thought, as a new study not only describes three new varieties but shows that they contain hundreds of genes seen in no other organism on the planet.
First discovered on a beach in Santiago, Chile, five years ago, giant viruses changed a lot of what we thought we knew about viruses. Now known as pandoraviruses, scientists had previously found bits of them floating around in the environment, but due to their enormous (relative) size, they assumed they must be a bacteria or archaea. Not so.
In 2013, it was shown that they were, in fact, massive viruses. While other giant viruses have been found before, pandoraviruses were unlike anything seen previously, with a genome almost double the size of the next largest family. A virus such as HIV exists using just nine genes, but pandoraviruses came in with a hefty 2,500 genes.
Despite being the same size as bacteria, and containing such a large genome, pandoraviruses can’t make their own proteins – for this, they need to infect amoeba. But if other viruses like HIV can exist with nine genes, and pandoraviruses don’t make their own cellular machinery, it then begs the question: why do they have such a large amount of genes?
The researchers decided to take a closer look at the viruses' genes to learn more, and what they found is quite weird. Their results are published in Nature Communications. It seems like the pandoraviruses have been independently going down their own branch of the evolutionary tree for quite some time.
Usually, the more closely related an organism is, the more genes they have in common. For example, while we may share upward of 96 percent of our genetic information with our closest living relative the chimpanzee, we only share around 60 percent with a fruit fly, and roughly 40 percent with a cabbage.
But the giant viruses appear to throw this out the window.
Genes that are found in one group or organism but not in any other are known as orphan genes. It turns out that the pandoraviruses are full of them, and despite having similar shapes and functions, each virus in the pandoravirus family only share around half of their genes with each other – the others were unique to the individuals.
Why this is the case, and what they actually do, is still anyone's guess. It seems they originate spontaneously within the viruses' genomes, making them the genetic oddity they are today.