Why viruses are not in our good books?

For all their amazing abilities of being able to survive outside their host, replicate in large numbers and still not be called 'living', viruses have failed to get some positive reviews for themselves. This is likely to be because the term virus is associated with some of the most scariest diseases in human history. Influenza, AIDS, SARS and the recent spread of Ebola, all have their roots in the term 'virus'. Yet, today, we will not dwell on how bad viruses are or the havoc they can cause. This post is more about how intelligent can viruses be! 

Parasitoid, Microplitis infecting larvae
Microplitis infecting its host worm.
Source: upclosephotography.blogspot.com
Although most viruses are associated with harmful diseases that are caused as a result of infection, there is also a large family of viruses who have a friendly association with their hosts. Called Polydnaviruses, these viruses have long been known to exist, mutually, with their hosts, the parasitoid wasps (which rely on smaller insects such as worms for their reproduction). On gaining maturity, the parasitoid wasps lays its eggs in the body of a living worm, where they incubate and hatch after the gestation period.

When the wasp lays eggs into the the worm, it is quite natural that the worm's immune system will detect a foreign body and act against it. While the wasp has no control over the immune system of the worm, it is the Polydnavirus, that it has injected, along with its eggs, that it depends upon to keep its eggs safe.

Polydnaviruses have just two main missions in their life cycle. 









1. Help its host infect the worm necessary for its survival.   


Polydnaviruses are capable to creating some unique proteins that can bring down the first line of defence in the worm body, the hemocytes. After infecting, the cells of the host worm, the virus quite generously chooses to only weaken the worm's immune system so that the wasp eggs can develop. This is in stark contrast to usual virus behaviour that we are quite used to seeing, where it overtakes the host cell machinery to make millions of its own  kind, ultimately resulting in the death of the host. 

Rather, Polydnaviruses also have some other tricks up their sleeve to help them combat the immune system. Not only can these viruses produce proteins that can inactivate attacks made upon the viral infection, they can also abort apoptosis (programmed cell death), the cell's self-destruct button to keep the infection from spreading further. While the virus seems to be putting in a lot of effort into safeguarding the interest of its friend, the parasitoid wasp, there is some selfishness behind this great act of generosity. The clue to this lies in the second aim of the polydnaviruses 

2. Ensure that it passes itself onto the next generation of its host.


To accomplish their second mission, Polydnaviruses have developed an interesting scheme of their own. Over generations of mutualism with their hosts, the Polydnaviruses have replicated their genome, broken them into smaller pieces (called proviral segments) and distributed them into genome of their respective hosts. During the process of reproduction, the host must first replicate its entire genome, and while doing so, also copies the pro viral segments. For every egg that the wasp makes, it also copies the complete viral genome into it and this is why the Polydnaviruses are so selfless after hijacking the worm immune system. For every egg that matures into an adult wasp, the virus makes it to the next generation, where it can repeat the same process and ensure its own survival. 

A recent publication Gaelen Burke et al at the University of Georgia and published in PLoS Genetics, has shed more light on how the process takes place. Using next generation sequencing technology, the researchers investigated the genome of a host, Microliptis demolitor and found some interesting facts about the pro viral segments of the M. demolitor bracovirus (MdBV) embedded in its genome. The researchers found that the pro viral segments were packed in the form of circular DNA segments and dispersed at at least 8 distinct locations in the host genome. All these segments were accompanied with unique sequences on either sides that would help them be recognized during reassembly. After reassembly, the virus exists as a provirus in each and every cell of its host but chooses to replicate only in the calyx cells in the ovaries of its host, where once again, it makes its way into the eggs and thus, the next generation. 

No wonder viruses have a hard time getting positive reviews!

Reference: 


Burke, G., Walden, K., Whitfield, J., Robertson, H., & Strand, M. (2014). Widespread Genome Reorganization of an Obligate Virus Mutualist PLoS Genetics, 10 (9) DOI: 10.1371/journal.pgen.1004660

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