Where did viruses come from?


Breaking down the microbiology world one bite at a time

Where did viruses come from?

Despite being invisible to our sight, viruses are everywhere. They can infect almost everything, plants, animals, bacteria and even other viruses … they cannot be escaped!  Yet, understanding their origin has been a long-lasting issue for the scientific community.  Viruses are indeed quite different from us. They are not made of cells but are more of an assembly of molecules, organized in a shape that protects the viral genes and allows the gene transport to a host cell. These genes are reduced to the bare minimum, i.e what allows the entry of the virus into the host cell, and what will constitute new viruses. Thus, viruses exploit the host cellular system to read their genes and to convert their information into new viruses. Because they are unable to proliferate by themselves, they were excluded from the “living world”. Classically, the living world is subdivided into three branches, related by an old common ancestor, with distinct features. Viruses do not belong to any and do not have their own branch. However, in 2003, the discovery of very special viruses, called Giant Viruses, has shaken scientists up. 

These particular viruses have a cell-like size and complexity, which is still invisible, but represent a huge difference at a viral scale (figure 1 for size comparison). Even more startling is that they seem to possess the machinery required to decipher their genetic information by themselves (see the article “Mimivirus – a giant in a small world.”). Those colossal viruses raise an important question among the scientific community: are all viruses part of a fourth and specific evolutive branch? In this hypothesis, they would have evolved from a cellular organism, then underwent a loss of cellular components and a genetic simplification. Or these giant viruses are the descendants of a viral-like non-cellular ancestor and would have gained components over time? 

Figure 1 : size comparison between two classical viruses (Adenovirus and Human Immunodeficiency Virus) and a giant virus (Mimiviruses family). Figure made with Servier Medical Art resources, adapted from Mitch Leslie, Science, 2017.

Dr Schulz’s research team in the USA tried to determine which hypothesis is the most likely to be true regarding viruses evolutive origin. They identified and described all the genes from a family of Giant Viruses, the Klosneuviruses. It allowed them to compare these genes with the ones of more classical viruses like the smallpox virus, then they used the similarities and differences identified between viruses to build the family tree of the Klosneuviruses. 

Then they also compared the giant viruses’ genes to those of their potential hosts. This step was done to determine whether the genes allowing the autonomous functioning of giant viruses are acquired from their cellular host, or if they are specific to these viruses. Schulz’s team study showed that the viral genes are really similar to algae’s ones. Algae being the main host of the Klosneuviruses studied. 

So all of these genome comparisons have excluded the hypothesis of a virus-specific fourth branch among living beings. On the contrary, they confirm that giant viruses are only an odd family among all the viruses. They just have been pilfering some genes from their host, thus increasing their genetic material and so their size!

Giant viruses are then unusual among their fellows, but they are unfortunately not the missing link to understand the origin of viruses in our world. One thing is for sure, viruses are plentiful, ubiquitous, and diverse, and we do not know what other strange virus we are likely to discover. 

Link to the original post: Giant viruses with an expanded complement of translation system component. Schulz et al. 2017. Science.

Featured image: (Megavirus from Wikimedia)