A new analysis lends weight to the hypothesis that viruses are living entities that share their evolutionary history with cells, researchers said Friday.
Scientists are divided as to whether viruses -- packets of DNA and RNA contained in a protein coat -- are living or not. Those who argue that they aren’t point out that they lack many of the features conventionally ascribed to living things, such as the ability to metabolize energy.
Even their reproduction requires a living host, as viruses lack ‘translational machinery,’ which is used to read and replicate DNA and RNA. Instead, viruses infect a cell and co-opt its own genetic machinery to reproduce themselves.
However, a new report by University of Illinois crop sciences and Carl R. Woese Institute for Genomic Biology professor Gustavo Caetano-Anollés and graduate student Arshan Nasir, which examines the genetic and evolutionary history of viruses, claims that they can prove they are living things.
The researchers said that part of the problem is the sheer diversity of viruses. The International Committee on the Taxonomy of Viruses recognizes seven orders of viruses, based on their shapes, sizes, genetic structure and reproductive methods.
"Under this classification, viral families belonging to the same order have likely diverged from a common ancestral virus," they said, according to a press release. "However, only 26 (of 104) viral families have been assigned to an order, and the evolutionary relationships of most of them remain unclear."
The researchers investigated their evolutionary history by examining the collection of protein structures embedded in all cells and viruses that are known as “folds.” These structures are the building blocks of more complex proteins, and by examining fold structures across different orders of living beings, the scientists could reconstruct an organism’s genetic history.
They examined 5,080 organisms from every branch of the tree of life, including 3,460 viruses. Their research revealed 442 protein folds that are shared between cells and viruses, and 66 that are unique to viruses. The results of their research were published in the journal Science Advances.
The data suggest that "a large number of viral genes are nothing like we have seen so far in the cellular world. They are most likely new genes created by viruses," the researchers said in their paper. They also suggested that not long after modern cellular life emerged, viruses evolved the ability to create a protective protein coat around themselves to safeguard their genetic payload, known as capsids.
"These capsids became more and more sophisticated with time, allowing viruses to become infectious to cells that had previously resisted them," Nasir said. "This is the hallmark of parasitism."
Caetano-Anollés said that many organisms require other organisms to live, and that viruses were no different. "Many organisms require other organisms to live, including bacteria that live inside cells, and fungi that engage in obligate parasitic relationships -- they rely on their hosts to complete their lifecycle," he said. "And this is what viruses do."