Giant viruses are degenerated higher cells, a new study suggests (2023)

Viruses were puzzling enough: are they a living thing? A collection of inert chemicals that can be replicated with help? A contagious crystal? Today they are purely parasites, but did their evolution necessarily follow other life forms?

Then, in 2003, giants were discovered in amoebas found in a water tower in England, and theories derailed. Mimiviruses, as they were later called, were so huge that they were initially mistaken for bacteria.

Mimiviruses are, among other things, parasites that attack single-celled organisms and whose genome is much more complicated than that of their smaller conspecifics. Out ofidentified as non-bacteria, and the realization that they are far more complex than other types of viruses have curious minds wondering what the heck they are and where they came from.

A research team from the Indian Institute of Technology in Bombay has now published a studymolecular biology and evolutionsuggesting that mimiviruses evolved from eukaryotes, i.e. complex cells with nuclei.

The molecular virologists Dr. Kiran Kondabagil and Dr. Supriya Patil created family trees for replication proteins in different life forms and realized that the genes of mimiviruses are more closely related to those of eukaryotes than to those of bacteria or small viruses.

There are several hypotheses for thisOrigin of Mimivirus' Kondabagil told Haaretz. His team is also not the first to point to a eukaryotic origin, he points out. But he and the team took a new approach, and their findings support this controversial theory of eukaryotic origin, he explains.

Their research also supports the hypothesis that mimiviruses are very old, in contrast to the competing theory that they evolved relatively recently from bacteriophages (viruses that attack bacteria), the team explains.

An illustration showing the size of giant viruses compared to normal bacteria and viruses.Credit: Meletios Verras/Shutterstock

DR. Kiran KondagagilPhoto credit: Dr. Kiran Kondagagil

when life was easier

Life can be divided into viruses, prokaryotes and eukaryotes, although it should be made clear that not everyone thinks viruses are a life form. Viruses generally consist of genetic material (RNA or DNA that encodes some genes) wrapped in a protein coat. Prokaryotes include bacteria and archaea (formerly known as archaebacteria); Your DNA is not enclosed in a cell nucleus but floats freely in the cell.

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Eukaryotes are everyone else, from amoebas to us: cells have nuclei that contain their DNA, except for their mitochondrial DNA.

That the Mimivirus has double-stranded DNA is neither here nor there: all eukaryotes have it, but so do bacteria, although in their case it is their only free-floating chromosome. Some viruses do this too, including our friend the Herpesviridae.

So basically viruses are the simplest form of life and we won't stop here to discuss whether they are life forms or replicating crystals, nor will we get bogged down in prions. Viruses generally don't have genes that control their own replication: they rely on the cells they attack to handle them. Outside the cell, they are dormant, which is why some insist they are not "alive." Bacteria and archaea are next in complexity: many are parasitic but able to reproduce.

The most complex are eukaryotes, nucleated cells, ranging from the amoeba to the palm tree and elephant. Many unicellular and all multicellular animals are eukaryotes.

Mimiviruses disrupt our neat little map of life by being viruses that have highly evolved genetic traits. The only surprise that emerged was that they have genes involved in DNA replication, transcription and even translation, if not the entire group.

No, the Mimivirus can't replicate, says Kondabagil; Because of this, it remains completely dependent on the host, but unlike its smaller relatives, it has a lot of equipment. That alone is an argument for demotion from a higher cell. This hypothesis further suggests that the mimivirus lost other genes encoding vital metabolic processes during evolution.

There is an unknown degeneration or a kind of reverse evolution. Take for examplemixed zoos, which are microscopic parasites consisting of one to a few cells, which turned out to be massive, to the great surprise of biologistsdegenerate jellyfish. At least one way reduces it that waylost his ability to breatheand deprives its host of oxygen. So this ability to start high and end low is available.

Mimivirus next to two virophages viewed through an electron microscope.Photo credits: Sarah Duponchel and Matthias G.


The new study is based on the properties of mimiviral DNA.

In general, previous work examined mimivirus-specific protein coding sequences consistent with those known elsewhere. Some genes matched bacteria, some archaea, and some eukaryotes. So looking at specific sequences that contain the coding for specific proteins wasn't very helpful, Kondabagil explains.

So he and Supriya looked at all of the mimivirus sequences involved in replication (when it infects a host) and found that most of them resembled those in eukaryotes and differed from the parallel mechanisms in bacteria, says he.

What about gene transfers, transposons, jumping genes, mechanisms by which genes are passed from one being to another? For example, resistance to antibiotics can be transferred from one bacterium to another, and they don't have to be of the same species, via "jumping genes". Could the ancestral Mimivirus have similarly preserved a set of eukaryotic genes?

It's not theoretically impossible, but unlikely, he replies. "We observed the entire replication process," explains Kondabagil. "A given virus cannot have borrowed the entire replication mechanism from somewhere else."

Thus, they realized that most of the enzymes involved in the replication mechanism of the mimivirus in question were more eukaryotic-like, not bacteria or archaea, which had evolved over time along with similar genes in eukaryotes.

They actively looked for gene transfers and concluded that three of the mimiviral proteins had been acquired from another source. But most of the code's representative components remained unchanged from the moment they diverged from the ancestor from which they arose, he explains.

In fact, genes for replication are generally conserved, says Kondabagil: That means they don't change (mutate) over time because mutations are lethal; Therefore, it is plausible that if a eukaryotic origin and a mimivirus progeny started with the same replication genes and were required for both, they would also remain similar over long periods of time.

They also discovered evidence that the proteins co-evolved, meaning they are linked into a larger protein complex with coordinated function.

A microscopic image of the tupanvirus, a member of the giant virus family.Photo credit: Jnatas Abraho

Taken together, these results imply that mimiviral's DNA replication machinery is ancient, the team explains, supporting the reduction hypothesis, an origin that began with a complex ancestor.

So is all this evidence that the Mimivirus originated as a eukaryote? It is not. dr Kondabagil expresses his conclusion with due caution: "We hypothesized that these mimuviruses could probably have a cellular origin," he says.

Alternatively, the Mimivirus could have started out as a virus and over time acquired genes from others.

billions of years

At the end of the day, Kondabagil explains, in bioinformatics you're trying to find explanations and figure out the minimum number of events for something to happen: the probability of something happening decreases with the number of events; All individual events are probabilities and highly inferential.

Why should anyone care about the origin of the Mimivirus? Because it's interesting, but there could be a pragmatic aspect. "Because these protozoa strongly influence the carbon turnover in the ocean, viruses play an important role in the ecology of our world. Therefore, studying them and their evolution is just as important as studying the viruses that cause disease,” he explains.

How old is the Mimivirus? Again, the jury is out, but his work leads him to suspect that the animal's last cellular ancestor, if there was one, lived around 3.5 billion years ago. We cannot even tell at this point whether the mimivirus family is monophyletic, descended from an original ancestor that probably arose before prokaryotes and eukaryotes separated; or polyphyletic, descended from several ancestral mimiviruses.

We'll probably never know the exact origin story, but not because we're not trying to figure it out.

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