How can the aids virus Evolve? [Archive]

Gilmar

31st January 2007, 12:40 PM

My somewhat educated and off-the-cuff guess about transmission would be "No, not likely". I'd guess it has to do with where the pathogen lives & reproduces, how it reproduces, what kind of coating it has, etc. For example, how do colds and tuberculosis live in humans and get released, vs. AIDS? What is the concentration of pathogen per liter of air or cm2 of skin? Is the question equivalent to "Could cats evolve to locomote like kangaroos or snakes", or is it much simpler than that?

Please, someone with more than my high-school-biology level of knowledge weigh in. I'm not sure why I'm even attempting to respond to this question, except it seems interesting and has been floating around in the back of my head and I'd like to know an actual answer myself.

Basilio

31st January 2007, 12:55 PM

Spot on, Gilmar.

It generally has to do with the virus family (Adenoviruses vs Togaviruses, for example) and the characteristics of their coats. It is possible that like some bacteria (my area) there may be "opportunistic" viruses which may have a narrow range of potential crossover hosts. Great study area though.

basilio

Ziggurat

31st January 2007, 03:15 PM

The Aids virus evolved and became able to infect humans when it was the SIV and could only infect chimpanzees. Could the aids virus evolve further and be transmitted by casual contact or become airborne? I don't know much about the aids virus. Are there any studies suggesting this is possible?

First off, viruses do not transmit through skin, so I'm not sure what you mean by "casual contact". Airborne viruses get transmitted through exhalation and inhalation, and do not travel through skin.

AIDS is a blood-born pathogen. It cannot survive outside of bodily fluids. In order to do so, it would have to radically alter its form. That is very unlikely to happen, especially in the time span we're interested in. But there's another issue to this, actually. AIDS is particularly nasty because it is almost invisible to the immune system, and so it doesn't really get attacked by the body's defenses. In order to radically alter its form to survive outside of bodily fluids, it would almost certainly need to incorporate different proteins in its outer coating. New proteins which could protect the virus in hasher environments could also be easy for the immune system to recognize as foreign, and make the virus susceptible to the body's own defenses. So an AIDS virus that mutated enough to become airborne might also not be very threatening. In fact, that would very likely be the case.

Furthermore, many pathogens (human and otherwise) show a tendency to evolve towards more benign forms over time. And the reason is fairly simple: pathogens which kill their host also prevent further transmission, whereas pathogens which do less harm to the host (for example, making you cough) can keep spreading around for a long time. The spread of the myxomatosis (http://en.wikipedia.org/wiki/Myxomatosis) virus (which infects rabbits) in Australia is a famous example where that process occurred very rapidly.

ClintonHammond

31st January 2007, 03:29 PM

Is it too inaccurate to suggest that a virus that kills its host is not a successful evolutionary strain?

Ziggurat

31st January 2007, 03:48 PM

Is it too inaccurate to suggest that a virus that kills its host is not a successful evolutionary strain?

That's an oversimplification. If the virus can manage to transmit successfully in the period before or after death, it can still be successful. But generally, the longer it leaves the host alive, the more time it has to retransmit, which is generally an advantage (though if the virus is too benign, it may not transmit well either - coughing and open sores, for example, can help transmission).

Roboramma

1st February 2007, 07:37 AM

Is it too inaccurate to suggest that a virus that kills its host is not a successful evolutionary strain?
I like to think of the parasite's interest in the host's lifespan in similar terms to the host's (gene's) interests in the host's lifespan.

There are two viable strategies (that I can think of) short lifespan, fast reproduction, long lifespan repetetive reproduction.

We humans have a long lifespan. We can continue to reproduce for years, and put of a lot of care in to each offspring.
Mice have (compared to us) a short lifespan. They produce a bunch of babies and then die pretty soon.

Neither of these stratgies is necessarily better. It depends on the organism and it's environment.

Back to parasites, sometimes it makes sense to keep the host alive for a long time so that it can have more of a chance of spreading the parasite, or spread it more often (just as in long-lived species, there's more of a chance of finding a mate, or of producing more often). On the other hand, keeping the host alive means not using up all of it's resources. It means not damaging it's health in ways that might help you spread.
So you can't give it bad diarhea for weeks (because it might die of dehydration for instance) even though you're likely to spread well that way. You have to be careful how you reproduce yourself within it's body, because it might waste away. Etc.

Sometimes one of these strategies is better than the other. In the environment that we've created for ourselves, of dense populations which make it easy to find a new host, the quick and virulent is more favoured than it was say ten thousand years ago.

Appologies if I've just found a longer way to say the same thing Ziggurat did.

Edit: Oh, and just because I think it's neat - the AIDS virus has evolved quite a bit since it first turned up in humans, and continues to now. I'm not sure, but I think Ken Miller talks a little about that in this video: http://www.youtube.com/watch?v=JVRsWAjvQSg. Been a while since I saw it though, so I might be misremembering, but it's a great video anyway.

Capsid

1st February 2007, 10:26 AM

SIV in apes and monkeys is not pathogenic/lethal. HIV is a relatively new zoonotic infection of humans and the interaction of host and virus has only just started.

sphenisc

1st February 2007, 10:55 AM

You might find this article interesting, particularly the section on Virulence

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1312209

Dustin Kesselberg

1st February 2007, 02:26 PM

Furthermore, many pathogens (human and otherwise) show a tendency to evolve towards more benign forms over time. And the reason is fairly simple: pathogens which kill their host also prevent further transmission, whereas pathogens which do less harm to the host (for example, making you cough) can keep spreading around for a long time. The spread of the myxomatosis (http://en.wikipedia.org/wiki/Myxomatosis) virus (which infects rabbits) in Australia is a famous example where that process occurred very rapidly.

That touches on another topic I posted several months ago. If viruses tend to wards more benign forms over time due to their inability to spread if they quickly kill off their hosts then why not viruses that not only become harmless but also become beneficial towards the host to help it survive so the virus can continue to have a host to live in? Most people in the thread I posted tended to think that it wouldn't be evolutionarily feasible though. But I still believe it's possible.

kleinjahr

2nd February 2007, 07:58 AM

"why not viruses that not only become harmless but also become beneficial"
Isn't that how our cells became eukarotes (sp?)?

Taffer

3rd February 2007, 04:37 AM

"why not viruses that not only become harmless but also become beneficial"
Isn't that how our cells became eukarotes (sp?)?

If you are meaning the likes of mitochondria and chloroplasts, the hypothesis is that they were bacteria-like organisms, no viruses.

If you meant something else, then nevermind...:blush:

dann

3rd February 2007, 08:18 AM

If viruses tend to wards more benign forms over time due to their inability to spread if they quickly kill off their hosts then why not viruses that not only become harmless but also become beneficial towards the host to help it survive so the virus can continue to have a host to live in? The idea is discussed in The Giving Plague (http://www.davidbrin.com/givingplague1.html) (fiction) by David Brin.

Mercutio

3rd February 2007, 08:47 AM

Also in the "Quarantine" episode of "Red Dwarf". Positive viruses for sex appeal, luck, etc.

skeptigirl

3rd February 2007, 04:54 PM

First off, viruses do not transmit through skin, so I'm not sure what you mean by "casual contact". Airborne viruses get transmitted through exhalation and inhalation, and do not travel through skin.....Viruses most certainly do infect skin. They cause warts and herpes to name two types of viral skin infections. And herpes can infect you on regular skin, not just the edge of epithelial and mucous membrane cells. Look up herpes and wrestlers.

skeptigirl

3rd February 2007, 05:03 PM

We know a lot more about viruses thanks to genetic science. Some recent work on influenza virus revealed a bit more about the nature of transmissibility.

Here's the answer in a nutshell. The means of transmission and route of entry for viral pathogens involve very specific and complex lines of DNA/RNA code. Mutations would have to be numerous and complex for a virus which was not airborne to become airborne. Give it a few thousand years and the right circumstances and it might be possible. In your lifetime, with HIV specifically, I wouldn't lose any sleep over it.

Now the H5N1 highly pathogenic avian flu virus on the other hand, needs as few as two nucleotide substitutions to become highly infectious in humans. It is already capable of airborne transmission. That's one you might want to keep an eye on.

skeptigirl

3rd February 2007, 05:17 PM

... If viruses tend to wards more benign forms over time due to their inability to spread if they quickly kill off their hosts then why not viruses that not only become harmless but also become beneficial towards the host to help it survive so the virus can continue to have a host to live in? Most people in the thread I posted tended to think that it wouldn't be evolutionarily feasible though. But I still believe it's possible.It may have happened. Viral DNA which merged and became part of an organism's genome could be passed on. But it would need to include infecting ova and/or sperm cells in sexually reproducing organisms. Otherwise the DNA/RNA wouldn't likely be passed on. That probably makes it unlikely in multicelled organisms. Gene transfer occurs via viral transmission in single celled organisms.

Here's a bit more on why it isn't likely in humans.
For an infection to provide the human host a benefit and therefore become a more common infection, the benefit would have to somehow affect natural selection of the person infected. And since the virus was already able to infect a person, providing a benefit to the human wouldn't likely mean the benefit was going to lead to more hosts for the virus or provide any other selection advantage for the virus. Maybe a virus that increased one's sex drive? ;)

I cannot think of any other traits a virus might provide that would result in more hosts or any other advantage for the virus.

Schneibster

3rd February 2007, 06:22 PM

Dustin Kesselberg

3rd February 2007, 08:14 PM

It may have happened. Viral DNA which merged and became part of an organism's genome could be passed on. But it would need to include infecting ova and/or sperm cells in sexually reproducing organisms. Otherwise the DNA/RNA wouldn't likely be passed on. That probably makes it unlikely in multicelled organisms. Gene transfer occurs via viral transmission in single celled organisms.

Here's a bit more on why it isn't likely in humans.
For an infection to provide the human host a benefit and therefore become a more common infection, the benefit would have to somehow affect natural selection of the person infected. And since the virus was already able to infect a person, providing a benefit to the human wouldn't likely mean the benefit was going to lead to more hosts for the virus or provide any other selection advantage for the virus. Maybe a virus that increased one's sex drive? ;)

I cannot think of any other traits a virus might provide that would result in more hosts or any other advantage for the virus.

How about if all the virus had to do to continue it's existence is to keep the human alive for as long as possible? Meaning somehow altering the individuals genome so that it lives longer or becomes better at surviving such as making it stronger or more intelligent or immune to other diseases. The 'population' of that virus could then live inside of that host for as long as it wanted. Not necessarily moving to other hosts.

Dustin Kesselberg

3rd February 2007, 08:16 PM

To directly answer the question in the thread's title, "by acquiring changes in its genetic sequencing." That's how all evolution of life forms on Earth occurs, at least since all life forms on Earth started using genes, which would be I guess about 3 or 4 billion years ago.

Aw, Really?

Capsid

4th February 2007, 03:47 AM

There are human endogenous retroviruses (HERV) which are integrated into the host genome and passed on from generation to generation.

This paper (http://www.blackwell-synergy.com/doi/pdf/10.1046/j.1365-3083.1998.00428.x?cookieSet=1)discusses the possible beneficial role of HERVs in influencing normal cellular processes including gene expression, polyadenylation, genomic recombination and possibly reproduction and cellular transformation.

skeptigirl

5th February 2007, 06:58 PM

How about if all the virus had to do to continue it's existence is to keep the human alive for as long as possible? Meaning somehow altering the individuals genome so that it lives longer or becomes better at surviving such as making it stronger or more intelligent or immune to other diseases. The 'population' of that virus could then live inside of that host for as long as it wanted. Not necessarily moving to other hosts.

You have to look at how the selection pressures would work, not merely speculate on some rationale.

Hypothesis:
Virus sheds for a lifetime, therefore the longer the organism lives the more the virus propagates.

A mutation sequence has to occur that allows the host to live longer. Such a mutation would have to offer a selection pressure in order for the mutation to spread. But it takes 80+ years for the host survival to give the mutated virus a selection advantage over a virus that didn't increase host longevity.

By that time the virus would have mutated many more times even if it did offer a selection advantage.

skeptigirl

5th February 2007, 06:59 PM

Dustin Kesselberg

5th February 2007, 10:43 PM

Such a mutation would have to offer a selection pressure in order for the mutation to spread.

Spread where?

But it takes 80+ years for the host survival to give the mutated virus a selection advantage over a virus that didn't increase host longevity.

Why?

Capsid

6th February 2007, 02:09 AM

Bad kink Capsid, it says my cookies are blocked but they aren't and other links work??????

Is this it? (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&list_uids=10469592)

"remnants of exogenous retroviruses that entered the germ line millions of years ago" and additional articles talk about some more recent additions to the human genome.

Interesting. Clearly more common than I would have imagined.
No that's not the paper. This is the article (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9790301&dopt=Abstract)abstract in PubMeD. I accessed the paper through a Google search of "beneficial endogenous retroviruses"; I can view the entire article including a pdf but I think that is because my work Institute has a subscription to Blackwell.

Cuddles

6th February 2007, 07:17 AM

skeptigirl

6th February 2007, 09:01 PM

It makes sense with the retroviruses. HIV is a retrovirus and it remains active throughout a pregnancy which is one way of merging with fetal DNA and becoming part of the genome from then on.

skeptigirl

6th February 2007, 09:25 PM

Spread where?

Why?Spread out as a mutated virus.

Do you know the basics of selection pressures, Dustin?

Basically, mutations occur at fairly steady rates. The organisms are reproducing at whatever rate the species reproduces at. If the mutation isn't a critical one, then it doesn't have to be selected to be passed on. It will just get passed on at the baseline reproduction rate.

Once a mutation is passed on, it continues through new generations indefinitely.

Then come 'selection pressures' acting on the genome. If a particular mutation provides a selection advantage, or if it happens to be along for the ride with other mutations that provide a selection advantage, then that mutation will end up in more organisms than mutations not favored by selection pressures.

All organisms in a species don't necessarily reproduce at the same rate. That's how the selection pressure results in specific traits being 'selected'. Thus the mutation 'spreads' among the future generations of the organism.

In order for a virus trait to be selected over other viruses without the trait, there has to be an advantage over the other viruses. In your scenario, you are asking why a virus doesn't get selected that offers a benefit to the host of a longer life, rather than an illness. A host would have to reproduce quickly for the benefit of living longer to have any selection advantage of the virus that was beneficial to the host. It just isn't going to happen in a human. How would a virus that resulted in longer human life have an advantage over a virus that didn't?

What does happen, though, is milder viruses will be selected over time. If a virus kills the host rapidly, it isn't passed on. The host that wasn't as ill spreads the virus. Eventually the milder viruses begin to dominate and the more lethal viruses die out.

Sometimes it's the host that is affected. As an infection kills people, the survivors reproduce. Eventually the survivors are more plentiful. That's what happened with the virus introduced to kill the imported rabbits in Australia. The virus killed about 95% of the rabbits. But the 5% that had better resistance, repopulated the country with virus resistant bunnies.

Deetee

7th February 2007, 10:30 AM

Don't forget about recombination, which happens frequently with wild type retroviruses like HIV.

Here (http://www.cdc.gov/ncidod/eid/vol3no3/burke.htm) is one article - there are other more recent ones, but it is the most useful in explaining the processes involved.

It is also possible to engineer a simian-human virus chimera (SHIV), which is basically an SIV with an HIV envelope and some HIV regulatory genes.

Schneibster

7th February 2007, 10:42 AM

Do you know the basics of selection pressures, Dustin?I doubt it. From what I've seen so far, Dustin doesn't "believe in" evilution.

Don't let me, however, stop you guys' interesting discussion on retroviruses. I've collected a couple new thought so far.

BTW, did you see what happened to kleinman on the "annoying creationist" thread? Someone who actually knows what they're talking about did an analysis of a paper that traces the evolution of the gene responsible for insulin back to its inception about 500 mya. You are all, I'm guessing, familiar with the creationist claim that "evilutionists" can't account for that gene? Well, looks like perhaps that's not particularly true. :D

Dustin Kesselberg

7th February 2007, 08:51 PM

Spread out as a mutated virus.

Do you know the basics of selection pressures, Dustin?

Basically, mutations occur at fairly steady rates. The organisms are reproducing at whatever rate the species reproduces at. If the mutation isn't a critical one, then it doesn't have to be selected to be passed on. It will just get passed on at the baseline reproduction rate.

Once a mutation is passed on, it continues through new generations indefinitely.

Then come 'selection pressures' acting on the genome. If a particular mutation provides a selection advantage, or if it happens to be along for the ride with other mutations that provide a selection advantage, then that mutation will end up in more organisms than mutations not favored by selection pressures.

All organisms in a species don't necessarily reproduce at the same rate. That's how the selection pressure results in specific traits being 'selected'. Thus the mutation 'spreads' among the future generations of the organism.

In order for a virus trait to be selected over other viruses without the trait, there has to be an advantage over the other viruses. In your scenario, you are asking why a virus doesn't get selected that offers a benefit to the host of a longer life, rather than an illness. A host would have to reproduce quickly for the benefit of living longer to have any selection advantage of the virus that was beneficial to the host. It just isn't going to happen in a human. How would a virus that resulted in longer human life have an advantage over a virus that didn't?

What does happen, though, is milder viruses will be selected over time. If a virus kills the host rapidly, it isn't passed on. The host that wasn't as ill spreads the virus. Eventually the milder viruses begin to dominate and the more lethal viruses die out.

Sometimes it's the host that is affected. As an infection kills people, the survivors reproduce. Eventually the survivors are more plentiful. That's what happened with the virus introduced to kill the imported rabbits in Australia. The virus killed about 95% of the rabbits. But the 5% that had better resistance, repopulated the country with virus resistant bunnies.

I think you're assuming the virus needs a "Selection pressure" to develop. Why can't it be engineered?

Dustin Kesselberg

7th February 2007, 08:52 PM

I doubt it. From what I've seen so far, Dustin doesn't "believe in" evilution.

What exactly have you 'seen' so far that would suggest I don't believe in evolution? I believe in evolution.

Capsid

8th February 2007, 02:44 AM

I think you're assuming the virus needs a "Selection pressure" to develop. Why can't it be engineered?
Live virus vaccines may be a case in point here. They have been attenuated (mutated) to replicate in the host without causing disease. This occurs by selection pressure by culturing the virus in vitro in specific conditions. Attenuation is done because it is not often obvious how to attenuate a virus by engineering and it's easier to culture than engineer.

skeptigirl

8th February 2007, 05:49 AM

I doubt it. From what I've seen so far, Dustin doesn't "believe in" evilution.

Don't let me, however, stop you guys' interesting discussion on retroviruses. I've collected a couple new thought so far.

BTW, did you see what happened to kleinman on the "annoying creationist" thread? Someone who actually knows what they're talking about did an analysis of a paper that traces the evolution of the gene responsible for insulin back to its inception about 500 mya. You are all, I'm guessing, familiar with the creationist claim that "evilutionists" can't account for that gene? Well, looks like perhaps that's not particularly true. :DLet me guess, after Kleinman saw the information, he ignored it and repeated his silly math won't work claim?

I didn't know Dustin was doing the same thing. I'd love for you to tell me Schn. is wrong, Dustin. But considering Schn. has a record of being right, then I have to say these kind of threads are as annoying as sales calls. The come on is a ruse.

You think someone has a genuine question about some mechanism of the evolutionary process but the person really just thinks they have some "Aha, see evolution can't explain [fill in the blank]." Since we have filled in fairly solidly all those blanks or "gaps" from 5 decades ago (seems the arguments are always based on science about that old) you take the time to explain the process for [fill in the blank] and the person who asked the question in the first place ignores the answer, continues to believe the world is flat, won't bother checking, fades away after insisting the answer was insufficient only to pop up again with a new ruse to ask again.

Amazing denial these people have. The world is round. Get on a plane and see for yourself, Dustin.

skeptigirl

8th February 2007, 05:52 AM

What exactly have you 'seen' so far that would suggest I don't believe in evolution? I believe in evolution.Oh, I should have read more before my last post. Nevermind, assume it applies to someone besides you Dustin and accept my apology.

skeptigirl

8th February 2007, 05:56 AM

I think you're assuming the virus needs a "Selection pressure" to develop. Why can't it be engineered?But I do wish you'd remember the train of thought here.

My answer was based on the question why viruses either made us ill or did nothing and why couldn't one "EVOLVE" to increase a person's life span rather than make the person ill.

Now you're off on something completely different.

They are using viruses to carry genetic material into people in hopes of repairing genetic deficiencies like cystic fibrosis. The research in is the early stages.

Schneibster

9th February 2007, 04:32 PM

Let me guess, after Kleinman saw the information, he ignored it and repeated his silly math won't work claim?No, he shifted the goalposts and then denied doing so. But good guess, nevertheless.

I didn't know Dustin was doing the same thing. I'd love for you to tell me Schn. is wrong, Dustin. But considering Schn. has a record of being right, then I have to say these kind of threads are as annoying as sales calls. The come on is a ruse.It was, however, an interesting conversation. Fortunately, it looks like events have proven me wrong.

You think someone has a genuine question about some mechanism of the evolutionary process but the person really just thinks they have some "Aha, see evolution can't explain [fill in the blank]." Since we have filled in fairly solidly all those blanks or "gaps" from 5 decades ago (seems the arguments are always based on science about that old) you take the time to explain the process for [fill in the blank] and the person who asked the question in the first place ignores the answer, continues to believe the world is flat, won't bother checking, fades away after insisting the answer was insufficient only to pop up again with a new ruse to ask again.Think of it like a game of "Whack-a-mole."

Schneibster

9th February 2007, 04:42 PM

What exactly have you 'seen' so far that would suggest I don't believe in evolution? I believe in evolution.Hmmm, I don't- I accept it as the most likely explanation. If you want something I believe in, I believe that freedom is more valuable than life itself.

You deserve at least some explanation. You asked a question, and I gave a direct answer. You expressed incredulity- at the notion that mutations are changes to the genome, or that life has been doing that here for a few billion years, the referent was unclear. Which ever it was, incredulity was perhaps not the correct response.

For what it's worth, I apologize.

Dustin Kesselberg

9th February 2007, 09:24 PM

Hmmm, I don't- I accept it as the most likely explanation. If you want something I believe in, I believe that freedom is more valuable than life itself.

You deserve at least some explanation. You asked a question, and I gave a direct answer. You expressed incredulity- at the notion that mutations are changes to the genome, or that life has been doing that here for a few billion years, the referent was unclear. Which ever it was, incredulity was perhaps not the correct response.

For what it's worth, I apologize.

Yea...The word you're looking for here is called "Sarcasm".

skeptigirl

10th February 2007, 01:05 AM

Dustin, wasn't your avatar a happy guy last time I looked?

Dustin Kesselberg

10th February 2007, 01:37 AM

Dustin, wasn't your avatar a happy guy last time I looked?

My last avatar was a Sunflower. Tournesol, The MediaWiki logo.

Schneibster

10th February 2007, 05:16 AM

Yea...The word you're looking for here is called "Sarcasm".So, you were being an a$$ and you got yourself into trouble. Bummer, dude. The suggestion I'd make is pretty obvious, I think.

Dustin Kesselberg

10th February 2007, 11:29 PM

So, you were being an a$$ and you got yourself into trouble. Bummer, dude. The suggestion I'd make is pretty obvious, I think.

I wasn't being an ass and I didn't get myself into trouble. I was being sarcastic and you took me literally.

Schneibster

11th February 2007, 12:19 AM

I wasn't being an ass and I didn't get myself into trouble. I was being sarcastic and you took me literally.Then don't whine about it.

Dustin Kesselberg

12th February 2007, 08:49 PM

Then don't whine about it.

I'm not.

Wolfman

12th February 2007, 10:13 PM

Skeptigirl,

I've quite enjoyed your comments, especially in regards to why a virus would be unlikely to evolve in humans to become beneficial to the host. This led me to a bit of a thought exercise, to consider how such a thing might actually work.

This led me to think of something like sickle-cell anemia; a genetic defect that causes health problems for the bearers, but nevertheless in certain areas of the world (especially Africa), it is nevertheless selected by evolutionary pressures because it also provides greater immunity against sleeping sickness.

So, what if we posited a virus that does harm to its host; but also provides greater protection against certain risks. Bear with me, I know that this is getting rather far into supposition here, but its a fun exercise.

I would see this as being most likely when two different viruses are involved. Assume, for example, that H5N1 mutates to successfully make the leap to humans. It has a high rate of transmission from host to host, a moderate mortality rate (ie. it is not likely to kill off its entire host population quickly), and a fairly rapid rate of mutation (similar to the AIDS virus).

Then we have another virus, something like a flu virus, which also has a high transmission rate, but minimal mortality. One particular mutation in this virus causes it to have a protein coating that is very similar to that of the H5N1 virus. We'll call this virus B1 (the original virus) and virus B2 (the mutated virus with a protein coating similar to that of H5N1)

Now, in populations that are infected first by the B2 virus, they will cause an auto-immune response which will prime the victim's immune system, resulting in those people being more capable of fighting off subsequent infections by the H5N1 virus. In populations that are infected by the B1 virus, the population will derive no such benefit, and many more will die off.

The result would be that as H5N1 spread, we'd find far larger populations of survivors who'd been infected with B2 than with B1; by providing a demonstrable benefit to human hosts, there would be strong evolutionary pressures for the B2 virus not only to eventually become much more dominant than the B1, but also to continue evolving/mutating as the H5N1 virus evolves/mutates, the two following a similar track of mutation.

Comments? :D

skeptigirl

13th February 2007, 02:36 AM

You are confusing a couple of issues WM that need more clarity. Most infections are limited and people do not acquire new genetic material with permanent resulting effects. There are exceptions. Some viruses are actively carried like HIV and hepatitis B & C. And a few become latent and are triggered to recur one or more times like the herpes simplex viruses and the human papilloma viruses.

You are talking about cross immunity in your example, a completely different thing. There are a number of pathogens with some cross-over immunity, usually to other organisms in the same family. Our immune responses are pretty specific and there isn't wide spread crossover in the antigen-antibody response mechanism.

Then there is a third thing and that is some genetic factor that affects susceptibility and resistance to infections. So sickle cell trait isn't pleasant but offers a benefit at the same time. There is some speculation that survivors of the plague in the middle ages may have included more people with what is called a CCR5 deletion. It affects the outer surface of white blood cells and appears to make it harder (though not impossible) for HIV infection to occur.

Your scenario sort of mixes these mechanisms up. I think if it were beneficial for your virus as you pose, it would be more common. But there just aren't any examples I can think of, so if it happens it must be rare. The hepatitis D virus only infects people who are carrying hepatitis B already. But that is the opposite of your idea.

However, we are a literal sea of infectious organisms that do not cause disease. And many of these organisms defend their territory which blocks infection with some pathogenic organisms. Again, I'm not aware of any viruses which do this. Maybe there are some I don't know about.

Wolfman

13th February 2007, 02:46 AM

However, we are a literal sea of infectious organisms that do not cause disease. And many of these organisms defend their territory which blocks infection with some pathogenic organisms. Again, I'm not aware of any viruses which do this. Maybe there are some I don't know about.
I think you misunderstood my intentions :-) I wasn't trying to claim that this actually does happen, or is much of a possibility. It was simply a thought exercise to consider a theoretical situation in which evolutionary pressures could cause a virus to develop that had a positive effect on humans. The "sickle cell anemia" thing was used to demonstrate a principle, not to create equivalence between the evolution of genetic diseases and viruses.

You had argued earlier that you could not envisage a situation in which a positive effect of a virus on a human's health (ie. you are healthier/more likely to survive with the virus than without) would cause that particular mutation to confer a greater advantage for survival/replication than other forms of the virus who didn't have that particular mutation.

I simply turned it into a thought exercise...to consider a situation where infection by one less lethal virus could protect people from infection by another, more lethal virus. In such a situation, at least theoretically, there would then be a strong competitive advantage for this virus...other forms of the virus that do not have this mutation will suffer negatively because their hosts are killed by the other, more virulent virus.

As to the likelihood of such a thing actually happening, I agree with you 100% that it is quite unlikely, and I certainly don't know of any examples of this.

Capsid

13th February 2007, 03:23 AM

I think you misunderstood my intentions :-) I wasn't trying to claim that this actually does happen, or is much of a possibility. It was simply a thought exercise to consider a theoretical situation in which evolutionary pressures could cause a virus to develop that had a positive effect on humans. The "sickle cell anemia" thing was used to demonstrate a principle, not to create equivalence between the evolution of genetic diseases and viruses.

You had argued earlier that you could not envisage a situation in which a positive effect of a virus on a human's health (ie. you are healthier/more likely to survive with the virus than without) would cause that particular mutation to confer a greater advantage for survival/replication than other forms of the virus who didn't have that particular mutation.

I simply turned it into a thought exercise...to consider a situation where infection by one less lethal virus could protect people from infection by another, more lethal virus. In such a situation, at least theoretically, there would then be a strong competitive advantage for this virus...other forms of the virus that do not have this mutation will suffer negatively because their hosts are killed by the other, more virulent virus.

As to the likelihood of such a thing actually happening, I agree with you 100% that it is quite unlikely, and I certainly don't know of any examples of this.
This does exist and is called viral interference, for example defective influenza virions (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=16621180&query_hl=2&itool=pubmed_docsum)may be produced which interfere with new infecting infuenza strains.

Schneibster

13th February 2007, 08:06 PM

Yea...The word you're looking for here is called "Sarcasm".is whining- you're implying that I was stupid for interpreting sarcasm as incredulity with bad punctuation.

Don't whine.

robinson

14th February 2007, 12:39 PM

Ziggurat

14th February 2007, 12:54 PM

Like cowpox and smallpox?

The cowpox/smallpox thing happens because when the body develops antibodies to cowpox, those same antibodies will also successfully defend against smallpox because the diseases are so similar. That's the cross-immunity skeptigirl refered to. But the cowpox virus itself does nothing to attack the smallpox virus, which is what I think skeptigirl was refering to when she talked about infectious organisms defending their territory.

robinson

14th February 2007, 12:57 PM

Dustin Kesselberg

15th February 2007, 05:07 AM

is whining- you're implying that I was stupid for interpreting sarcasm as incredulity with bad punctuation.

Don't whine.

It's not whining. It's a simple statement.

robinson

15th February 2007, 09:03 AM

Schneibster, Dustin, you can use chat or private messages to give each other a reach around.

blutoski

15th February 2007, 11:48 PM

Aha. I see that. An interesting subject. Antibiotics were discovered just because of that ability of one micro-organism to kill others, or defend its territory. I don't think a virus does this. Thanks.

This also happens 'by accident'. The most familiar example is that human vaginal flora make this tissue surface inhospitiable to fungi. However, when a woman takes certain types of antibiotics, vaginal bacteria die back, and there is an increased risk of yeast infection.

I believe there is also a tendency for viral-infected cells to alter their surfaces so much that they become uninfectbile. Superinfection (cellular infection with more than one virus) is still possible, but rare.

blutoski

15th February 2007, 11:58 PM

It may have happened. Viral DNA which merged and became part of an organism's genome could be passed on. But it would need to include infecting ova and/or sperm cells in sexually reproducing organisms. Otherwise the DNA/RNA wouldn't likely be passed on. That probably makes it unlikely in multicelled organisms. Gene transfer occurs via viral transmission in single celled organisms.

Here's a bit more on why it isn't likely in humans.
For an infection to provide the human host a benefit and therefore become a more common infection, the benefit would have to somehow affect natural selection of the person infected. And since the virus was already able to infect a person, providing a benefit to the human wouldn't likely mean the benefit was going to lead to more hosts for the virus or provide any other selection advantage for the virus. Maybe a virus that increased one's sex drive? ;)

I cannot think of any other traits a virus might provide that would result in more hosts or any other advantage for the virus.

This is really two questions:

1. Have viral genomes ever merged with host genome? Answer: yes. Endogenous retroviruses are not only factual, but are actually a valuable arrow in the evolution evidence quiver.

2. Have viruses ever become symbiotes? Answer: mehhh, pooossiblyyy. The value of endogenous retroviruses is that they replicate inside the host genome, and when they get it 'wrong', they can replicate host genome itself. This adds bucketloads of raw DNA scaffolding, from which de novo mutations can arise, over and above single point-mutations. Retrotransposons may also have survived because of this benefit, although I don't think there's evidence they're related. eg: ALU family.