"James O. McInerney Ph.D." <jamm at nhm.ac.uk> wrote:
>Hi all,
>> It has been pointed out to me by somebody here at the museum that the
>date for the mitochondrial endosymbiosis (or whatever you wish to call
>it) has continuously been pushed further down the eukaryotic tree (by
>unearthing evidence for secondary mitochondrial loss).
>> Now, there is general agreement that the mitochondrion is monophyletic
>(descendent from the ancestors of Alpha proteobacteria). Alpha
>proteobacteria are _relatively_ recent bacteria (as judged by molceular
>trees, and assuming a reasonable molecular clock), so mitochondrial
>eukaryotes can only have evolved _relatively_ recently. Therefore, a
>HUGE length of time elapsed between the splitting of 'Eukarya' and
>Bacteria. Sooo, in that time there must have been some eukaryotes that
>were successful.
>>My points are:
>>1. Were the eukaryotes that survived for > 1,000,000,000 years,
>completely supplanted when the mitochondrion evolved? maybe it is a
>problem with our sampling of microeukaryote diversity (quite likely
>actually), maybe they WERE supplanted, maybe they didn't exist at all.
I would argue this point backwards. The descrepancy between the
branchlength of alpha-proteobacteria connecting them to other
proteobacteria is far smaller (3-4x for most 16S trees I've seen)
than the branchlengths of the mitochondrial eukaryotes (trichomonads
and up). This implies that the molecular clock is likely inaccurate-
varying by 3-4x in rates between eukaryotes and bacteria.
The long branch connecting eukaryotes to prokaryotes I believe
is likely indicative of a period of accelerated evolution of
the ribosomal RNA due to changes in constraints (a result of
the huge change from prokaryotic to eukaryotic translation
machinery). I don't believe it means anything at all in
terms of time. Molecular clock extrapolations based on
rRNA, I believe will prove entirely erroneous when it comes
to eukaryote-prokaryote divergences.
>>2. From the energy point of view (hope my ignorance of bio. doesn't
>embarrass me here), if these primitively amitochondrial eukaryotes had
>good metabolic systems (they lived on their own for maybe a billion yrs,
>didn't they?), then these systems should surely be seen at least in some
>phylogenetic groups (one would think) today.
>>> I guess the problem is that less than 100 genes are sequenced in
>Giardia, less than 30 in Trichomonas and not many genes from the others.
>Not many people are PCRing primitive eukaryotic 18S rRNA sequences from
>natural samples (not to the same extent that they are doing it for
>prokaryotes) and until then....
Actually, I believe that Norm Pace's group is looking for such
things quite vigourously....
And don't forget that there are 2 amitochondrial protist
phyla that no one even seems to be able to hold in
stable culture-- the oxymonads and retortamonads. There
could be life in the Archezoa hypothesis yet.
Cheers
Andrew J. Roger
---
