The status of viruses has been a controversial point in biology, which hits at the very core of how we define life. According to most definitions of life, viruses are not alive – they are bits of DNA or RNA that invade cells to reproduce.
The largest of these organisms, known as giant viruses, seemed to blur the line between being alive and not. However, a new study, published in Science, suggests that these viruses are just lucky opportunists rather than new members of the “being alive club.”
The researchers, led by Dr Frederik Schulz, studied four new related giant viruses discovered in an Austrian waste treatment plant. These Klosneuviruses appear not to have evolved their extensive genome from an ancestor, but to have stolen genes from hosts.
The team looked at the family tree of these viruses. The Klosneuviruses show an incredible ability to gain new genes. One of these giant viruses (part of the Mimiviridae family) was able to produce enzymes that had the ability to interact with 19 amino acids, which are the building blocks of proteins.
One of the staples of cellular life as we definite it is the ability to assemble proteins. The common viruses are incapable of this feat of biology and they need to invade and use living cells to reproduce. But giant viruses have shown this capability, so some scientists had hoped to welcome them into the domains of living things.
At least in the case of Klosenuviruses, the ability is not due to evolution but to exploitation. Originally, these organisms were much smaller, but they became this big due to a frenzy of gene stealing, rather than through adaptation from a common ancestor. The researchers showed that several enzymes seem to have emerged independently in different Mimiviridae at different times, another hint of this genetic pillaging.
"This finding strongly supports a scenario in which giant viruses emerged from much smaller viruses after acquisition of many genes from diverse cellular hosts, potentially over long time periods. In brief, Our data supports the ‘bag of genes’ scenario where a smaller virus infected different eukaryote hosts and picked up genes from these divergent hosts over time," Schulz told IFLScience
There are three branches of life according to the Woese model – bacteria, archaea, and eukaryotes (which include us, plants, fungi, and so on). Giant viruses have been proposed as a fourth branch, but this research suggests their application might be rejected.