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Diploid Viruses?

Ian A. York york at mbcrr.dfci.harvard.edu
Thu Oct 27 13:04:24 EST 1994


In article <38oj6c$es6 at hermod.uio.no> ohungnes at bioslave.uio.no (Olav Hungnes) writes:
>
>Yes, many of the viruses produce lots of noninfectious particles and are 
>in this respect not particularly effective. On the other hand, it isn't 
>very important for them to be so energy-efficient, parasites as they are. 
>They also usually do not compete with other viruses, a setting in which 
>the more efficient would prevail; their battle is mainly with their host. 
>Like many other parasites they make a very large number of offspring, 
>each of them having a rather low probability of success. Viruses sure 
>could reproduce faithfully if it was vary important for them, it 
>just isn't. Ensuring genetic variation, e.g. to escape the immune 
>response is more urgent.

	I think I used the wrong word, or at least a word with the wrong 
implications, when I said "inefficient".  I was responding to the earlier 
poster (sorry, can't remember who) who used "efficient" and implied that 
this meant clean reproduction with tight control.  (I may have 
overinterpreted the post, but that was how I read it.)  My point was that 
this was not the case - that viruses work with what they have, under 
severe constraints, and sometimes these constraints force sloppiness into 
their system.  In other words, the strategy of error-prone but rapid and 
high-copy-number replication may well be more efficient for the virus 
than a more controlled replication, since the control mechanisms might 
take up room in the viral genome that is better used for something else.  
Thus efficiency for the virus, is not necessarily the most efficient 
strategy in a less constrained system.  The same applies to other factors 
in viral infections.  
	Another point arising from your post is the role of 
non-infectious mutant particles as diversions in an immune response.  I 
don't agree that this is a valid "reason" for non-infectious particles.  
After all, if you have the choice of producing 2000 infectious particles, 
or 20 infectious particles and 18000 non-infectious, either way you 
produce the same diversions for the immune system, but in the former case 
you have 10x as many clones potentially able to take advantage of the 
potential distraction.  
	This is different from the retrovirus mutation argument; there I 
agree that there is at least an argument to be made that this helps 
evade the immune response.  However, this depends at least partially on 
the mutants being viable, so as to take advantage of the evading 
mutation, right?  
	Another case in point is hepatitis B.  Here the vast number of 
non-infectious particles are probably chaff for the immune system, but 
importantly these do not contain genomes; the investment in these 
particles is relatively low.  So the non-infectious diversion doesn't 
really apply here.
 	Again - many of the viruses that have non-infectious particles 
are not particularly persistent - flu, and I think picornaviruses, for 
example.  In these cases, it is easier to think of the approach as 
blasting through a replication cycle as fast as possible - forget the 
editing - just to get through the amplification before the immune system 
kicks in fully.  
	Finally, there was an interesting paper in PNAS (I think) a few 
months ago about the advantages of mutations in viruses related to their 
methods of immune evasion.  Can't remember the details, but if anyone 
cares I can pull it out and post it.

Ian
-- 
Ian York   (york at mbcrr.harvard.edu)
Dana-Farber Cancer Institute, 44 Binney St., Boston MA 02115
Phone (617)-632-4328     Fax  (617)-632-2627




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