Caution, giant viruses from Mars! is that so?

The headlines of the news sites screamed: Viruses from Mars hitched a ride on a meteorite. The scientists didn't even claim it

In the latest issue of The prestigious scientific journal Science A fascinating discovery has been made. French researchers discovered a huge virus, ten times larger than an average virus and with a huge genetic load of about 2,500 genes - much more than an average virus. And if that's not enough, this virus shares only about seven percent of its genetic information with other organisms known to man - a very low rate.

The sensational publication gave rise to pretentious and sensational headlines in the press such as: "The biggest virus in the world was discovered: it arrived here from Mars” (mako), “The invasion from Mars: the biggest virus in the world - from space" (ynet) "Assessment: They "caught a ride" from Mars on a meteor” (NRG). These titles are very far from reality and more suitable for a second rate science fiction movie. I would like to start by saying that according to the available information it is impossible to conclude that the virus came from Mars, or from any other place outside the Earth, what's more, the researchers do not even hint at any such possibility.

In the article before us we will try to understand how the researchers discovered the virus and identified it, why it is unlikely that it originated from outside the earth and what is the real importance of the discovery.

So what was in the study?
As part of the study, the researchers were able to isolate two giant viruses - one from a freshwater reservoir in Australia and the other from the depths of the sea near Chile. They cleaned the samples they collected from the soil and looked for evidence of viral activity. Since as part of its life cycle the virus multiplies inside the host until the host can no longer contain it and explodes, the researchers looked for evidence of exploded cells. In the suspicious samples they found large particles suspected to be viruses, and after isolating them they compared their structure using an electron microscope and saw that the structure of the two viruses was similar but their protein composition was different. From this they concluded that these are viruses of different species.

In order to verify that they were indeed viruses, the researchers soaked them together with amoebae (a type of single-celled organism) and followed their life cycle for 15-10 hours. What was found was that the amoebas ingest the viruses, but unlike normal food particles, the viruses made holes in the particles they were trapped in, and instead of the amoeba digesting them, they released their contents into its intracellular fluid. After 4-2 hours the nucleus - the organelle that contains the amoeba's genetic information - began to undergo changes, 8-6 hours later the amoeba lost its normal shape and became a spherical structure, and eventually it disintegrated and about a hundred new viruses were released and went out to infect new amoebas. Such behavior is suitable for a viral life cycle, as can be seen in the viral outbreak cycle video.

At this point, the researchers could determine with a high degree of confidence that it was indeed a virus and not a single-celled organism such as bacteria or yeast. Now the researchers turned to characterizing the viruses, and for that they decoded their genetic code and compared it with other viruses. It turned out that viruses have a respectable amount of genes that is 2,556-1,502 - much more than what is needed for an average virus (the minimum necessary for a virus is 4 genes). This figure may indicate a certain complexity in the life cycle of the virus.

When the researchers compared the genetic material of the two viruses with other viruses, it was discovered that they shared only about seven percent of the genetic material. This means that 93 percent of the viruses' genes are unique to them, and this is indeed a sensational discovery. This is exactly the point where people may suspect that this is a virus that is not from this world, due to the great difference between it and other viruses.

However, among the non-unique genes (seven percent) were found genes that encode key enzymes such as replication of the hereditary material, packaging, core enzymes, etc. In other words, the enzymes important for the life cycle in the new virus are similar to the corresponding enzymes in other viruses. The researchers also showed that the virus operates according to the same standard genetic code typical of all living things in the world, from this it follows that it is probably a local virus that did not come from another planet.

Later, the researchers wanted to check whether it was a specially developed virus or perhaps we have here a primitive single-celled creature that degenerated and adopted the properties of a virus. To this end, they looked for key enzymes in it that are essential for the basic life processes of
the bacterium, including structural enzymes and enzymes that deal with energy production, but they did not find any of them, so it seems likely
It is very likely that this is an evolved virus and not a degenerate bacterium. However, they found in the virus genes from bacteria and more developed single-celled organisms, which the virus apparently attached to while inside the amoeba.

So is he from Mars or not?

As we have seen, a new virus has indeed been discovered that apparently has very little in common with other viruses, but still the basic processes of its life cycle are very similar to known terrestrial viruses - including structural similarity at the enzyme level. The virus works according to the same basic genetic code according to which all living things in the world work. He needs a host to reproduce. Without a host, he would not be able to reproduce himself and would not be able to undergo an evolution that would allow him to adapt to his environment.

If we assume for a moment that a virus arrives on Earth on an asteroid, this virus must find a way to reproduce in order to survive and develop, and the likelihood that the place from which it originally came has amoebas like on Earth is extremely slim. All of this leads to the conclusion that despite all the differences between this virus and other known viruses, it probably still originated on Earth and there is no justification for the bombastic and misleading headlines that appeared on the news sites.

Why is the virus so different?
It is important to note that this is not the first time that a virus with a high percentage of unique genes has been found. Another example is the giant viruses (Mimivirus), which also contain a very high percentage of unique genes. This phenomenon has several possible explanations: one is that those 93 percent of different genes are "specialist genes" that help the virus adapt to the host as a result of an arms race in which the amoeba develops defense mechanisms and the virus bypasses them. This possibility is not particularly likely, because evolution does not create enzymes out of thin air - the change is gradual, so there must be at least a faint similarity between it and other proteins.

Another possibility is that it is a very distant relative of the viruses we know today - a remnant of an ancient virus, and perhaps even a remnant of an intermediate vertebrate between the first life and bacteria. This is the possibility that the researchers take into account and intend to investigate in depth, because if it is indeed true, it is a particularly sensational discovery from which we can learn a lot about the development of life.

Another possibility is related to the climate prevailing inside the amoeba. The amoeba is a large single-celled creature that swallows almost anything it meets - viruses, bacteria, algae, yeast and more. Inside the amoeba, their particles break down and mix, and it is likely that parts of genes from other creatures have mixed with the genes of the virus. It is possible that they gave him an advantage and were preserved, it is possible that these are parts of genes that did not give him an advantage but also did not harm him and remained in him as inactive genes, or that do not contribute to the virus. Such a mechanism, called lateral genetic transfer, is a known mechanism that is not unique to this virus. We also know it from the swine flu virus.

A final possibility is that we simply do not know enough viruses yet to draw a family tree of virus evolution. It is possible that many more viruses from this family are hidden in the depths of the sea and that they are much less rare than we think, it is very possible that we have not yet discovered the tip of the iceberg, and today when we have more developed research tools than before we can uncover a whole world of microscopic creatures.
Whatever the reason, the real importance of the discovery is not the origin of the virus, but how it works. We have hundreds of new enzymes here that are unknown to science and are just waiting for their mechanism of action to be deciphered. who knows? Maybe a technological development or even a medicine will grow from here.

Thanks to Dr. Yael Motzafi from the Department of Structural Biology for his assistance in preparing the article

Gerty Cedar
PhD student, Department of Biological Chemistry, Weizmann Institute of Science
Editor of the "Science Database" website of the Davidson Institute