Benign Viruses

[infiniteregress]

As i have usually heard it in lecture, viruses are 'obligate intracellular parasites', which depend explicitly on cellular nuclei for reproduction. My understanding is that the reproductive action of a virus requires the destruction of the host cell. This spells doom for single-celled organisms, although not necessarily for large populations of them, such as biofilms, or in our case, as multicellular organisms.

 

However i've always had a little voice of doubt in the back of my mind on this point, I've wondered whether there was any kind of 'benign' virus which was capable of interacting with it's host without being so destructive (and therefore high profile).

 

Usually when i bring up 'positive' roles for viruses, i hear about viral transduction, such as in laboratory settings, but also as implicated in horizontal transfer events 'in the wild'. Nevertheless my understanding of viral transduction is that it requires a pathogenic stage in order to first acquire genes from the donor organism before transmitting them to the recipient. For the sake of clarity, in this sense 'pathogenic' refers not just to causing disease in humans or other animals(and plants also, i suppose), but also in bacteria, which i guess scientists dont usually consider capable of 'getting sick'.

 

At any rate, such a virus would need to enter the cell, uncoat, hijack enzymes to replicate new viral dna(or rna, as the case may be), as well as ribosomes to produce the appropriate capsids and then exit the cell without causing more harm than the host cell can repair, certainly without lysing the cell. This sounds quite exceptional, barring a very clever solution of course, which nature has provided before.

 

The point of this post is to ask any resident experts whether there is evidence of such a thing existing in nature. If not thanks anyway.

[Mr Skeptic]

Some viruses lyse the cell open to leave, killing it. Some viruses leave by embedding themselves in a "bubble" of the cell's membrane which then splits off; this is called budding, and is used especially by enveloped viruses. Some viruses are proviruses; they embed themselves into the host cell's DNA and are replicated with the cell; eventually they will leave the cells by lysis or budding. None of this says much about how nasty the virus is or whether it might be beneficial.

 

For example, HIV is an enveloped retrovirus (a type of provirus). It doesn't kill cells by lysis, and embeds itself in the cell's DNA. Nevertheless, it is not really beneficial despite being theoretically capable of transferring genes and its not killing of its host cell. (in fact, the immune system tries to kill off any cells that are infected with a virus, they can after all be replaced easily enough). On the other hand, I've heard that some of the embedded fragments of viral DNA in our genomes might have some use, and sometimes getting infected by one virus such as cowpox can provide immunity to other viruses such as smallpox which are far nastier.

[CharonY]

Well theoretically there is very little potential for active viruses that are beneficial. The reason is less in their destructive nature but due to the fact that their genomes are already extremely strained just by the necessary genes to replicate. In fact, they stack their genes and make use of slippery transcription to express them all. In order for them to be beneficial as such they would have to introduce new genes or alleles, for which there is little room. The only exception is the mentioned transduction event. While the result is a gene transfer event, usually it also renders the virus inactive (as part of the genes necessary for viral replication are replaced).

 

However, integration of the virus into genomes themselves can lead to mutations that over long or short can result in a fitness increase. During longer co-evolution the viruses tend to become inactive, or at least less destructive. In terms of fitness from the viewpoint of the virus you can imagine it like this:

- a virus that eventually kills its host and spreads aggressively will proliferate in short-term, but may run out of hosts

- a virus that spreads passively and eventually becomes fixed in the genome will have maximum spread in the population but it takes longer