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Viruses and Evolution

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Viruses and Evolution

Postby rimblock » Mon Dec 14, 2009 10:22 am

Hiology,
I have a couple of questions i was hoping someone could answer...

1) Does the massive human population we have nowadays mean there is more chance for some virus to mutate and wipe us all out, or does it give us some sort of advantage?
2) Are there any viruses that affect all living things regardless of their species? And if not, why not, since something like that would surely have evolved first (back when all living things were a lot more closely related and had less genetic differences)?

Thanks in advance.
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Postby biohazard » Mon Dec 14, 2009 10:45 am

1) Even though big populations make huge pandemics possible, they also offer a vast diversity in our genetic code. Basically this means that there is a chance of some virus wiping out a big portion of the human race, but not all of us, since some people are almost certain to carry a protective mutation against that given virus. We can already see this with the HIV: in some regions protective mutations to the prevalent HIV strain have emerged, though in very few individuals so far. But if the pressure is big enough, these people will be likely to increase in numbers.

If human populations were small, huge wiping-out pandemics could not happen. Before the start of agriculture, global or even continent-wide epidemics did not really happen.

2) As far as I know, there is no known virus that could infect all living things. To the contrary, vast majority of viruses are very host-specific and target cell-specific, and cannot normally cause disease in any other organisms. Some notable exceptions include viruses that cause the so-called zoonoses, which are animal diseases that can in some cases contract to humans (this can happen the other way round as well). However, viruses causing zoonoses are "poorly designed" to live in other organisms than their normal hosts and thus often cause too fulminant infections, sometimes even killing the new host. But considering the vast amount of virus species/strains there are, only a small fraction of them can cause cross-species infections.

Bacteria that cause zoonoses are more common, because many bacteria are typically far less species-specific than viruses are. Being exclusively parasites, viruses target very specific structures (often receptors) on their target cell's surface that make them unable to infect cells that lack the given target. Pathogenic bacteria, to the contrary, do not depend as much on such specific targets and thus can have a wider spectrum of hosts, or they can cause opportunistic infections even if they normally were free-living non-pathogens in the soil or in some animal's gut etc.

When life was very young on this planet and organisms much more uniform in function there might have been more wide-spectrum viruses or virus-like particles.
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Postby rimblock » Mon Dec 14, 2009 10:55 am

Biohazard,
Thanks for your reply.

Do you not think it is strange that eventough the DNA we all share (all animals) must have been around the longest, it does not come under attack from some ancient virus. It seems like there is a 'gap' in the evolution of viruses?!

Obviously if it did exist, we probably wouldn't(!), but that doesn't really answer the question of why it seems to be missing..!

Thanks again.
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Re:

Postby biohazard » Mon Dec 14, 2009 11:04 am

rimblock wrote:Biohazard,
Thanks for your reply.

Do you not think it is strange that eventough the DNA we all share (all animals) must have been around the longest, it does not come under attack from some ancient virus. It seems like there is a 'gap' in the evolution of viruses?!

Obviously if it did exist, we probably wouldn't(!), but that doesn't really answer the question of why it seems to be missing..!

Thanks again.


The "gap" is probably the fact that there is no virus that would have a means of entry to all possible host cells in the world (because of the diversity in target receptors and such). Also, cells have many ways of destroying foreign DNA that manages to enter it, and this all-infecting virus would have to evade all such mechanisms as well. Finally, viruses do not really attack the DNA in cells anyway, even though some species can integrate into it.

Curiously though, it seems that we all carry viruses or viral DNA in our genomes. In theory it is possible that some piece of the DNA of most of the organisms could be originally an integrated virus. Just as we carry mitochondria (and plants chloroplasts) that were originally prokaryotes, there could be viral DNA that actually encoded something useful for their host cell and so has remained in place for as long as mitochondria - millions of years.
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Postby rimblock » Mon Dec 14, 2009 11:05 am

I guess what i am saying is 'are only certain parts of our DNA susceptible to viruses', and what point in evolution did those immune parts develop? And do viruses that exist today target 'old dna' or 'new dna'?!

Thanks again for your lengthly (which is good) response :0)
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Re: Re:

Postby rimblock » Mon Dec 14, 2009 11:15 am

biohazard wrote:
rimblock wrote:Biohazard,
Thanks for your reply.

Do you not think it is strange that eventough the DNA we all share (all animals) must have been around the longest, it does not come under attack from some ancient virus. It seems like there is a 'gap' in the evolution of viruses?!

Obviously if it did exist, we probably wouldn't(!), but that doesn't really answer the question of why it seems to be missing..!

Thanks again.


The "gap" is probably the fact that there is no virus that would have a means of entry to all possible host cells in the world (because of the diversity in target receptors and such). Also, cells have many ways of destroying foreign DNA that manages to enter it, and this all-infecting virus would have to evade all such mechanisms as well. Finally, viruses do not really attack the DNA in cells anyway, even though some species can integrate into it.

Curiously though, it seems that we all carry viruses or viral DNA in our genomes. In theory it is possible that some piece of the DNA of most of the organisms could be originally an integrated virus. Just as we carry mitochondria (and plants chloroplasts) that were originally prokaryotes, there could be viral DNA that actually encoded something useful for their host cell and so has remained in place for as long as mitochondria - millions of years.


The mitochondria thing is some crazy sh*t! I like what you are saying about some viruses encoding useful traits into hosts - i can easily imagine some sort of virus making its host immune to other viruses, and that trait being passed on to the host's offspring. It would be like 'survival of the fittest', but for viruses. Which leads me to another question..! Assuming that their ultimate goal is to perpetuate their DNA sequence, do you think encoding their identity into another organism is a satisfactory survival technique! It was good enough for the mitochondria!
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Re: Re:

Postby biohazard » Mon Dec 14, 2009 11:39 am

rimblock wrote:
The mitochondria thing is some crazy sh*t! I like what you are saying about some viruses encoding useful traits into hosts - i can easily imagine some sort of virus making its host immune to other viruses, and that trait being passed on to the host's offspring. It would be like 'survival of the fittest', but for viruses. Which leads me to another question..! Assuming that their ultimate goal is to perpetuate their DNA sequence, do you think encoding their identity into another organism is a satisfactory survival technique! It was good enough for the mitochondria!


Yeah, that is what I meant. I do not know if this has been proved or disproved scientifically, but at least at the moment I do not see why it could not happen.

But again, viruses generally do not target DNA. They enter the cell's cytoplasm and express their genome there, producing building blocks for new viruses just as if they were the cell's own molecules. Only some viruses integrate into the host genome, and I believe the main function of that is to remain dormant and protected from outside threats such as viral-DNA-degrading enzymes of the host cell or (nowadays) antiviral drugs. These viruses then sometimes re-activate and start producing new viruses.

That being said, it is also a convenient way for the virus to replicate its genome if it is integrated into a dividing cell. But only in the germ line cells the viral genome can be transferred along with the host from generation to the next. I am not aware if any of the currently known viruses actively seek to infect germ line cells and integrate into their genomes in order to multiply their DNA along with the host's. The idea is interesting, though.
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Postby rimblock » Mon Dec 14, 2009 12:07 pm

Cool - thanks!

I didn't appreciate (even though you mentioned it several times!) that all viruses do not behave the same way - ie the don't completely take over a cell.

Once again, your time and knowledge is much appreciated.
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Postby kolean » Mon Dec 14, 2009 1:45 pm

Just curious, what is the theory on the origin of viruses? Remnants of prokaryotic DNA?
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Re: Viruses and Evolution

Postby jonmoulton » Mon Dec 14, 2009 4:49 pm

There is a virus required for formation of placenta in sheep. The virus helps the cells fuse to form a syncytium.

Dunlap KA, Palmarini M, Varela M, Burghardt RC, Hayashi K, Farmer JL, Spencer TE. Endogenous retroviruses regulate periimplantation placental growth and differentiation. Proc Natl Acad Sci U S A. 2006 Sep 26;103(39):14390-5. Epub 2006 Sep 15.

There is a similar virus required in humans -- I can get a reference if anyone wants to follow up on that.
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Postby kolean » Mon Dec 14, 2009 7:41 pm

Very interesting reading. I have not come across endogenous retrovirus in my developmental studies. Then again, I have not studied developmental biology for so long to know what is beyond trophoblast/placental/extraembryonic material.
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Re: Viruses and Evolution

Postby biohazard » Tue Dec 15, 2009 7:30 am

jonmoulton wrote:There is a virus required for formation of placenta in sheep. The virus helps the cells fuse to form a syncytium.

Dunlap KA, Palmarini M, Varela M, Burghardt RC, Hayashi K, Farmer JL, Spencer TE. Endogenous retroviruses regulate periimplantation placental growth and differentiation. Proc Natl Acad Sci U S A. 2006 Sep 26;103(39):14390-5. Epub 2006 Sep 15.

There is a similar virus required in humans -- I can get a reference if anyone wants to follow up on that.


Wow, that's some interesting stuff. I knew we had viral DNA in our genomes, but that they could actually benefit us was just my own guess. I am very pleased to notcie that my guess wasn't all that bad - some viruses are beneficial for their hosts in many interesting ways :)
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