The discovery of new species of giant viruses of the genus Pandoravirus leads to a new hypothesis on their evolution

Pandoravirus Quercus (Image courtesy IGS- CNRS/AMU. All rights reserved)
Pandoravirus Quercus (Image courtesy IGS- CNRS/AMU. All rights reserved)

An article published in the journal “Nature Communications” describes a research on Pandoraviruses, giant viruses that raised perplexity among scientists since the discovery of the first species, announced in 2013. A team of researchers discovered three new species belonging to the same family and proposed an explanation for their amount of genes, really huge for viruses, claiming that they create them on their own.

After the discovery of the first giant viruses, many other species have been identified, which show a number of similarities and differences that convinced scientists to create a separate families. Pandoraviruses were classified into the Pandoraviridae family and the discoverers of the first species, Jean-Michel Claverie and Chantal Aberge, together with other researchers from Structural and Genomic Information Laboratory (CNRS/Aix-Marseille Université), Large Scale Biology Laboratory (CEA/Inserm/Université Grenoble-Alpes) and CEA-Genoscope, continued their research.

One result was the identification of new species of Pandoraviruses in various parts of the world. Pandoravirus quercus was isolated in the soil of Marseilles, France, Pandoroavirus neocaledonia was isolated in brackish waters near the airport of Noumea, in New Caledonia, and Pandoravirus macleodensis was isolated in the fresh waters of a pond near Melbourne, in Australia, only 700 meters away from the place where the researchers isolated the species Pandoravirus dulcis a few years ago.

These new giant viruses were replicated in the laboratory using amoebae of the species Acanthamoeba castellanii as hosts. This made it possible to study their replication cycles under electron microscope together with those of the species Pandoravirus dulcis and Pandoravirus salinus, the first of this family discovered. The researchers also analyzed the DNA of new species to compare them with those of the already known species, including Pandoravirus inopinatum.

The analyzes showed that the various Pandoraviruses share about half of the genes that encode proteins, a low percentage compared to the norm in members of the same family. The new species discovered also show a large number of orphan genes, meaning genes that encode proteins that don’t have equivalents in other living organisms, a characteristic in common with the species already known.

Another anomaly is that the orphan genes were different for each Pandoravirus species. This means that these are genes are much less likely to have been inherited from common ancestors, a factor to be kept in mind when discussing the origins of giant viruses in general.

The use of bioinformatics, that is of informatiion technology applied to biology, allowed to perform more in-depth analyzes of the various species of Pandoraviruses’ genetic characteristics. Various comparisons and statistical analyzes related to those genomes suggested to the researchers that those orphan genes were created by the viruses on their own.

The researchers themselves are aware of the problems related to the hypothesis of the creation of new genes. First of all, the new proteins created thanks to those genes should have certain characteristics and usefulness to Pandoraviruses. Basically, it’s an interesting hypothesis to explain the enormous size of these viruses but still far from proven.

According to the researchers, they need to find new species of the family Pandoraviridae, also to understand the possible relationship with other giant viruses such as Mollivirus sibericum, discovered in Siberia in 2015, again by Jean-Michel Claverie and Chantal Aberge. That’s a virus still not classified in any family, another confirmation of the problems that scientists have despite the genetic analyzes performed on the various species.

Every new research on giant viruses and each new species discovered adds new information that could be crucial to better understand their origins and their evolution but sometimes all this brings new questions instead of new answers. For a few years, the discussions were about the possibility that these viruses were “degenerate” cells that became parasites or descendants of viruses that acquired genes from the organisms they attacked. Now the hypothesis of the creation of new genes has been proposed and it’s possible that the correct answer is different for the various families of giant viruses.

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