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HIV-1 vs HIV-2

Patrick O'Neil patrick at corona
Wed Mar 15 17:18:23 EST 1995

On 15 Mar 1995, RYBICKI, ED wrote:

> ...
> > The most parsimonious scenario of all has an HIV ancestor 
> incubating > within human populations for about a thousand years or 
> so, with jumps > into various simian lines within the last few 
> centuries.
> Whaaaaaaaat?  Says who?!  All of a sudden, HIV has been in HUMANS 
> for a thousand odd years, and gets BACK into simians??
> Surely a most parsimonious explanation is that HIVs separated in 
> simians, and went merrily along until humans were infected in recent 
> times?  Far mor eparsimonious than postualting all of the jumps into 
> monkeys that would have been necessary to acccount for all of the 
> SIVs,

Not really.  More jumps are required, based on sequence homologies, if 
one starts with SIV (as seen in chimps) and then assumes a jump to humans.

Just because there are a number of SIV variants doesn't mean that they 
all had to individually jump from human to each.  It is more likely that 
simians spread the virus amongst themselves rather than humans 
individually producing a jump into each.  

As for a century or more period of time evolving in humans, this is not a 
big problem either.  If it started out rare and rather benign as is the 
case with most retroviruses, then it could lay about for any period of 
time until some series of mutations and selective pressures began to 
favor a more pathological form (high sexual intercourse rates within and 
between populations is certainly going to favor higher pathogenicity.  

> several of which are quite divergent.  And some of which, I 
> might point out, don't cause immunodeficiency disease in their 
> natural hosts - indicating a better adaptation to these than to 
> other hosts, such as lab monkeys.

Low pathogenicity within a host doesn't necessarily a "better" adaption. 
If selective pressures within a natural host population favor rapid
transmission and pathogenesis, then it will develop in that direction
regardless of how long the virus has been floating around within the host

Low pathogenicity within the host might just as well indicate a recent 
move into that population, meaning that the virus hasn't quite developed 
for maximum efficiency within that host, just as an overly high 
pathogenicity could mean the same thing IF it reduces the rate of 
successful transmission.

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