Beneficial Viruses

Due to the insights gained from a decade’s worth of viral research, it’s not surprising that in the autumn of 2013, Penn State University offered a newly developed course in viral ecology. For a century, the word virus had been exclusively associated with certain human, animal, plant or computer diseases. Virus, which is rooted in the Latin word for a slimy, poisonous liquid, had nothing but negative connotations. Yet virologists have slowly come to realize that only a minority of viruses are virulent. Most are innocuous, some are definitely beneficial, and others are indispensable to their hosts. In light of this, as pointed out in a review of mutualistic viruses, a part of the old definition of a virus is incorrect:

intracellular parasites with nucleic acid,capable of directing their own replication, that do not serve any essential function for their host, have an extrachromosomal phase and are not cells.

Mutualism, as opposed to parasitism and commensalism, is a form of symbiosis that benefits two (or more) organisms. Given that viruses are not cells, it may seem odd that the term mutualistic is used to describe some of them, but some mutual symbioses between plant and fungi would not exist without the existence of a virus, and in other cases the living host and virus both directly benefit from their relationship.

Let’s survey some of the more colorful examples from a variety of life’s kingdoms.

1. Bacteria
Many bacteria integrate, within their own DNA, the entire nucleic acid sequence(shown in green in diagram below) of a virus(blue). These dormant (lysogenic) viruses protect bacteria from other forms (lytic) of viruses that could burst out and kill their hosts. Other bacteria(red) that do not carry viral genome are not protected from free viruses.

2. Insects
a) Wasps
Several species of parasitic wasps lay their eggs in living hosts. It’s surprising that the immune system of the victim does not encapsulate and kill the foreign egg. In the lepitdopteran caterpillar, it is actually a mutualistic polydnavirus (class I virus: dsDNA) carried by the wasp which prevents encapsulation and keeps wasp eggs thriving within caterpillar hosts.















b) Aphids In the lab, rosy-appled aphids(A )that were free of virus did not develop wings.

Only those infected by dysaphis plantaginea densovirus, a class II ssDNA virus,  (see B and C in two stages of development) grew wings. Interestingly, if the aphids were infected with rosy apple virus, they remained wingless. Flight helps the host and its viral guest move from one branch or tree to another.

3. Ménage à Trois Among Plants, Fungi and Viruses

In the hot spring environment of Yellowstone National Park, certain grasses (Dichanthelium lanuginosum) can withstand extremely hot soil. Using thermal soil simulators, researchers kept plants in soil at 65°C for 10 hours and
37°C for 14 hours per day for two weeks. The grasses only survived if they were colonized by the fungus Curvularia protuberata, which in turn had to serve as the host for a third mutualistic partner, a virus. In the diagram, Wt= wild type, An= virus-infected in lab, VF= virus-killed, and NS= non-symbiotic plant.

























4. Animals (specifically mammals)
About 6 years ago, the media reported that in sheep, some retroviruses (class VI: ssRNA-RT) related to Jaagsiekte sheep retrovirus are critical during the early phase of pregnancy when the placenta begins to develop. But more generally, retroviruses may have played a key role in the evolution of the placenta.

The envelope (env) genes of retroviruses function to promote fusion of the viral membrane with the plasma membrane of a host cell. Syncytins are derived from envgenes and are expressed in the placenta, where they promote fusion of cytotrophoblasts with the syncytiotrophoblast. Thus far six syncytin genes have been discovered including two in the mouse and two in higher primates. These genes are not orthologous so each represents an independent capture from a retrovirus. Yet another example of convergent evolution! There is more. The envelope protein of retroviruses is immunosuppressive and endogenous env genes may contribute to immune tolerance by the mother of the fetal semi-allograft.

http://placentalevolution.blogspot.ca/2013/02/endogenous-retroviruses-and-placenta.html

Whereas a rice plant is protected from drought when infected with cucumber mosaic virus, mice infected with lymphotropic viruses do not get type I diabetes. Investigators have no doubt hit upon the tip of a mind-boggling iceberg of viral relationships, which have even more profound implications in a world of genetic engineering.

Other Sources:

 

 

 

  •   Roossinck M. The good viruses: viral mutualistic symbioses. Nature Reviews Microbiology 9, 99-108 (February 2011) | doi:10.1038/nrmicro2491
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One thought on “Beneficial Viruses

  1. I find that most of these discussions always take a top-down approach which makes these relationships baffling. In my view, I try to examine it from the organism’s perspective.

    A virus or a bacteria has no concept of the organism it is inhabiting. It is merely the environment in which it lives. So, like most organisms it will modify its environment to make it more suitable for its own survival. While there is much emphasis on evolution, the reality is that organisms cannot wait for evolution to solve their daily survival problems. Evolution can describe long-term transitions, but it does nothing for an organism in the present.

    As a result, we can see these organisms interact with one another, and [in some cases] initiate changes that would normally have been attributed to “random mutations”. Instead, these are traits that become adaptive and if they improve fitness, ultimately selectable.

    Success breeds success.

    Taken to its logical conclusion, it could well change our perspectives on disease, by suggesting that it is merely the result of incompatible actions taken in a hostile environment. In short, these are the “failures” that decrease fitness and risk extinction.

    The only reason such viruses or bacteria don’t go extinct, is because they have so many environmental options to choose from, but I don’t believe it’s a coincidence that, given the opportunity, most viruses and bacteria will “develop” the means to cross species barriers.

    If you’ll permit the loose analogy, it’s like our human voyages of exploration. One never knows when one finds a better environment to exploit.

    Liked by 1 person

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