Quasispecies

Brian Foley btf at t10.lanl.gov
Thu Oct 30 14:24:18 EST 1997


Joe Felsenstein wrote:
> 
> In article <637ntb$s0t at net.bio.net>, toni  <toniv at iname.com> 
> wrote:
> >Nobody talks about quasispecies?
> 
> The flip answer is "no, they don't".  The more interesting 
> answer is "should they?"  It is important to talk about 
> genetic variation in species.  When someone working on 
> humans or Drosophila does this, they say they are talking 
> about genetic variation in natural populations.
> When a virologist talks about it, they say they are 
> talking about quasispecies, which sounds infinitely more 
> mysterious and novel.

	I think the important difference, which may apply to
prokaryotes as well as viruses, is that most eukaryotic species
exchange genetic material through sexual recombination and this
greatly influences the designation of "species" and tends to
keep the total gene pool within the species well-mixed.  Most 
viruses and prokaryotes do not have a diploid genome and sexual
recombination to keep the gene pool mixed.  
	If we accept the definition of "species" as "a group
of individual which can mate and produce viable offspring", which
is quite a useful definition in terms of population genetics
of eukaryotes, what are we then to say of asexually reproducing
organisms?  The term "quasispecies" can be useful, as the virus
particles within one host have a good chance of exchanging 
genetic material with one another (by two virions entering
the same cell), whereas virions physically isolated in different
hosts have no chance of exchanging genetic material.

> 
> So the question is, is anything gained by considering it as a
> discussion of "quasispecies" rather than "genetic variation"?

	Perhaps, if we knew more about how to apply the
"species" and "strain" labels to asexual organisms, we would
have a better handle on this.  Bacteria can exchange genetic
material via plasmids and phages.  Not all bacterial "species"
that can exchange material with one another can be usefully
classified as belonging to the same species.  If we find a plasmid
that can carry genes between gram-positive and gram-negative
bacteria, we do not want to just lump them all into one "species"
for that reason alone.
	For phylogenetic analysis, it might be nice if organisms
were either truly asexual with no genetic recombination at
all, or truly sexual with diploid genomes split 50:50 at each
mating.   The in-between state, in which there is a variable amount
of gene flow between lineages, makes things a bit tricky.

> 
> --
> Joe Felsenstein         joe at genetics.washington.edu
>  Dept. of Genetics, Univ. of Washington, Box 357360, Seattle, WA 98195-7360 USA

-- 
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|Brian T. Foley               btf at t10.lanl.gov                       |
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