Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!throop
From: throop@cs.utexas.edu (David Throop)
Newsgroups: sci.bio
Subject: Sex and organelles (was: chloroplasts)
Message-ID: <1179@ai.cs.utexas.edu>
Date: 21 Jan 91 16:46:17 GMT
References: <845@frc.frc.maf.govt.nz> <22722@well.sf.ca.us>
Organization: Dept of Computer Sciences, UTexas, Austin
Lines: 23

While we're on the subject of chlorplasts and other organelles, let me
ask, "Why do we still have them?"
   The DNA in the organelles is separate from the DNA of the rest of
the genome and each individual inherits all of its organelle DNA from
just one parent.  As such, the the organelle DNA gets none of the
advantages of sex.  These advantages include:
   Diploidy - having two copies of genes, with minor variations - if
some environmental factor makes one copy disfuntional the other copy
may still function and let the organism survive.
   Genome shuffling - the chance to take beneficial mutations from two
parents and combine them into a single individual.
   And there are other advantages.  [My terminology may be off, I'm
not a biologist.]
  
    So if sex is so great why don't organelles have it?  Specifically,
why has there not been evolutionary pressure to move the functions of
the organelle out of the organelles' DNA onto the chromosomes?  Even
if the organelles originated from symbiotes, why haven't the two
genetic legacies been joined over evolutionary time?

David Throop