Genome Length

R M Bernstein ralph at ccit.arizona.edu
Fri Oct 6 12:31:07 EST 1995

In Article <453bfj$qtm at ccshst05.cs.uoguelph.ca>, tjackson at uoguelph.ca
(Timothy R Jackson) wrote:
>the salmonids are a good example of the rough idea that you're getting 
>at.  about 25-100 million years ago their ancestor doubled their genome 
>which gave rise to the family salmonidae (this is the same process that 
>is thought to have given rise to the vertebrates).  their closest ancestor, 
>smelts, have half the genetic material of salmonids.  the salmonids are 
>a highly diverse taxa with an extensive range, expoiting a wide variety 
>of environments and exhibiting varying life history behaviours.  this may 
>be a result of the "evolutionary potential" that an extra set of genes 
>gave them.

and someone else wrote:

:Genome size varies not by a few kb worth of genes but by orders of magnitude 
:between organisms which can otherwise appear very similar.  Whole books have 
:been  written on this topic (see for example, Cavalier-Smith, The 
Evolution of 
:Genome Size, John Wiley & Sons ltd, 1985).  The idea of whether this extra DNA 
:has a function is still debated. But we do know that most of this extra DNA
:is not genes (we know this from old hybridization experiments and from the
:genome sequencing projects, which are now giving us a glimpse at the gene
:density & organization in widely varying organisms such as yeast and human). 

    these are both great answers, but in disacord with them is the example
of xenopus, which 20-30 or so mya underwent a pseudopolydiploidy event. 
(the time is approx, i cant really rem)  this is a strange beast, with some
genes being duplicated 1.5 times.  what purpose this serves is unknown to
me.  but i would speculate along your lines that the duplication is selected
for, -look how sucessful x.laveus is in the lab!- mayhaps because of some
extra potential that allowed the existing stuff to be altererd.  plains
    in the case of the shark, which have a human or bigger sized genome,
they may use alot of it for ig genes.  many of their genes are "fused" in
the germiline, necessatating thousands, tens of thousands, whatever, copies
of functional genes and their regulatory regions.  these, when cloned, are
on ave 
5 kb.  so 5kb X 1000 or 10000 or more copies?  alot of space "wasted" (?) or
taken up.  this may have allowed the shark to duplicate the hell out of
these things, because they have alot of strange ig molecules not yet
identified in higher vertebrates.

hope to have added,
regards, ralph

Ralph M. Bernstein
Dept of Micro/Immuno
University of Arizona
Ph: 602 626 2585
Fx: 602 626 2100
url: http://lamprey.medmicro.arizona.edu

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