DNA Helps Trace Origin of Giant Virus

With the discovery of the first giant virus in 2003, the year was marked as a big year for virologists. As the characteristics of a virus add to the difficulties in defining them, they do not fit comfortably in the existing tree of life and thereby inciting the curiosity of many people. The evolution of viruses have a have long been a topic of interest among the scientific community. It especially holds when it comes to giant viruses (very common in oceans and other water bodies) that can produce new viruses with very little help from the host, in contrast to most small viruses (which utilize the host's machinery to replicate).

"Because these single-celled organisms greatly influence the carbon turnover in the ocean, the viruses have an important role in our world's ecology. So, it is just as important to study them and their evolution, as it is to study the disease-causing viruses," says Dr. Kiran Kondabagil, a molecular virologist at the Indian Institute of Technology (IIT) Bombay.

Evolution of the Giant Virus

A series of analyses were performed on the major genes and proteins involved in the DNA replication machinery of Mimivirus, the first group of giant viruses to be identified to determine their evolution, by a study published in the journal Molecular Biology and Evolution.

The team was set to examine which of two major suggestions regarding Mimivirus evolution, the reduction and the virus-first hypotheses, were more supported by their results.

The reduction hypothesis suggests that the giant viruses emerged from unicellular organisms and shed genes over time; the virus-first hypothesis suggests that they were around before single-celled organisms and gained genes, instead.

The study team created a phylogenetic tree with replication proteins and a technique called multidimensional scaling to determine how similar the Mimiviral proteins are. It was found that those from Mimivirus were more closely related to eukaryotes than to bacteria or small viruses.

A greater similarity would indicate that the proteins co-evolved, which means that they are linked together in a larger protein complex with coordinated function. And indeed, their findings showed greater similarity.

Genetic Selection of Mimivirus

The team also revealed that genes related to DNA replication are similar to and fall under purifying selection, which is natural selection that removes harmful gene variants, constraining the genes and preventing their sequences from varying. Such a phenomenon typically occurs when the genes are involved in essential functions (like DNA replication) in an organism.

These observations imply that Mimiviral DNA replication machinery is ancient and evolved over a long period, thereby in favor of the reduction hypothesis. Thus the DNA replication machinery already existed in a unicellular ancestor, and the giant viruses were formed after getting rid of other structures in the ancestor, leaving only replication-related parts of the genome.

"Our findings are very exciting because they inform how life on earth has evolved. Because these giant viruses probably predate the diversification of the unicellular ancestor into bacteria, archaea, and eukaryotes, they should have had major influence on the subsequent evolutionary trajectory of eukaryotes, which are their hosts. An increased understanding of the mechanisms by which viruses copy themselves and self-assemble means we could potentially modify these viruses to replicate genes we want or create nanobots based on how the viruses function. The possibilities are far-reaching!¡¨ says Dr. Kondabagil

The study thus anticipates that these findings could lay the groundwork for translational research into technology like genetic engineering and nanotechnology.

Source-Medindia