Re: Why don't mitochondria have junk DNA?


John Harshman wrote:
> rev.goetz wrote:
>
> > John Harshman wrote:
> >
> >>rev.goetz wrote:
> >>
> >>
> >>>John Harshman wrote:
> >>>
> >>>
> >>>>Selection for fast replication? That's the usual idea for bacteria, but
> >>>>do mitochondria have to replicate that often compared to the nucleus of
> >>>>the same cell? That would depend on the lifetime of a mitochondrion and
> >>>>its standing population in the cell. I'll have to look that up.
> >>>>
> >>>>If not that, then what?
> >>>>
> >>>>There are mitochondria with junk. In fact the control region has been
> >>>>duplicated several times in different groups of birds, and one of the
> >>>>copies is clearly non-functional. So it does happen. But why so rarely?
> >>>
> >>>
> >>>Junk DNA is an extavagant byproduct of evolutionary processes because
> >>>the rate of neutral mutational insertions from various types of
> >>>repeated sequences appears to be significantly more frequent than the
> >>>fixation rate of all mutational deletions. And I recall that only
> >>>diploid cells have the mechanisms that generate various types of
> >>>repeated sequences. (I do not have the time to look up the reasons for
> >>>this.)
> >>
> >>You would seem to be talking about unequal crossing over. That's only
> >>one of a host of processes that generate new sequences.
> >
> > Unequal crossing over is only one of a few mechanisms that help to
> > generate non-coding DNA in eukaryotes.
>
> It's the only one I know of that relies on diploidy.
>
> >>>Concerning some junk DNA in mitochondria, this could have occurred by
> >>>DNA transfer from the nuclear genome to the mitochondrial genome. I am
> >>>not sure if I have heard of cases of DNA transfer from the nuclear
> >>>genome to the mitochondrial genome, but I know that I heard of several
> >>>examples of gene transfer from the mitochondrial genome to the the
> >>>nuclear genome. So I would not be surprised to see if the reverse ever
> >>>happened.
> >>
> >>I don't know of any such case. Like I said, the only mt junk I know of
> >>involves a duplication of a mitochondrial region.
> >
> > I missed that you said it was a duplication of a mitochondrial region,
> > so scratch what I said about DNA transfer from the nuclear genome to
> > the mitochondrial genome.
> >
> > In general, mutational insertions/duplications are more likely to occur
> > in eukaryotes compared to prokaryotes. And as someone said earlier in
> > this topic, many eukaryotes are not harmed by an accumulation of DNA in
> > the genome. So many insertions/duplications are neutral in terms of
> > fitness in many eukaryotes, but in many prokaryotes a large increase in
> > genome size causes a decrease in fitness. So natural selection prevents
> > a large increase of "useless DNA"in many prokaryotes.
>
> As I theorized at the outset. But can this possibly apply to
> mitochondria? Larry's mechanism obviously can't, since mt genomes are
> tiny by comparison with bacterial genomes. The other suggested
> mechanism, selection for speedy replication, also would seem not to be
> that strong in mitochondria, though that would depend on their
> population genetics. I really have to dig this up and read it again:
> Birky, C. W., Jr. 1991. Evolution and population genetics of organelle
> genes: Mechanisms and models. Pages 112-134 In R. K. Selander, A. G.
> Clark and T. S. Whittam (eds), Evolution at the Molecular Level. Sinauer
> Assoc., Sunderland, MA.

Even though mitochondria has a small genome size compared to
prokaryotes, natural selection favoring a compact genome could still be
the primary force that keeps it compact. For example, we know by theory
that mitochondria ancestors once had a complete prokaryote genome when
they were prokaryotes. And I conjecture that natural selection favoring
compactness helped to reduce the mitochondrial genomes to their current
sizes. And I see no reason why anything else but natural selection
favoring compactness would be needed to keep the mitochondrial genomes
near their current sizes.

On the otherhand, I know nothing about "selection for speedy
replication" so I cannot comment about that.

.

Relevant Pages

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