James O. McInerney wrote:
>Jonathan Badger wrote:
>> So you are saying that protists with a non-mitochondrial plastid, had
>> mitochondria, lost them, did perfectly fine without any plastids for
>> a while, and *then* picked up non-mitochondrial plastids at a later
What do plastids have to do with it? They derive from a completely
unrelated endosymbiotic event to that which gave rise to mitochondria.
>Nothing so elaborate. Simply, some protists such as Trichomonas
>vaginalis do not have mitochondria at the moment. Their ancestors
>clearly HAD mitochondria (Horner et al., Proc. Roy. Soc. Lond. B series.
>1996) but the extant species does not have one. This is not the only
>example, but to split protists up on the basis of
>mitochondrion-containing and mitochondrion-lacking is not a proper
>phylogenetic division either.
I would like to point out that it is not as clear cut a situation as this
explanation would indicate. One has to distinguish between gene phylogeny
and organelle phylogeny. Yes, the genes studied by Horner et al, Bui et al
(PNAS 93, 9651), Roger et al and Germot et al (both PNAS in press) are
most closely related to those of the bacterial lineage leading to
mitochondria (the genes are those for chaperonins - cpn60, mt-hsp70 and
cpn10, incidentally). However, there are several different scenarios that
would explain how trichomonads have acquired the genes:
1. The ancestor of the trichomonads had mitochondria as stated by
McInerney (post-mitochondrial common ancestor for the genes).
2. The ancestor of trichomonads had the bacterial endosymbiont that gave
rise to mitochondria in other eukaryotes but the lineage diverged before
the organelle as we know it became established (post-symbiosis but pre-
mitochondrial common ancestor for the genes).
3. The ancestor of trichomonads entered into a symbiosis with a bacterium
closely related to that which eventually was to give rise to mitochondria
(pre-symbiosis common ancestor for the genes).
4. The ancestor of trichomonads acquired the gene from an endosymbiotic
eukaryote that had mitochondria, no trace of which now remains (secondary
endosymbiosis origin for the genes).
5. The ancestor of trichomonads acquired the genes by lateral transfer
from a bacterium closely related to the one which gave rise to
mitochondria in other eukaryotes but without any symbiosis being involved.
I also believe that scenarios 1 and 2 are the most likely but I do not
believe the data allow us to state that trichomonad 'ancestors clearly
HAD mitochondria'. The presence of a double membrane bound organelle in
trichomonads (the hydrogenosome) makes such statements tempting but
caution is needed. In other cases the situation is much clearer - certain
amitochondrial eukaryotes have close relatives that contain the organelle,
implying secondary loss. Indeed, I am starting to believe that there may
not be ANY primitively amitochondrial eukaryotes known!
On a related topic, I seem to recall a theory stating that the nuclear
membrane arose in response to the acquisition of the mitochondrial
endosymbiont, to partition the two genomes. Can anyone remind me where
I saw this in print?
C. Graham Clark, Ph.D.
Department of Medical Parasitology,
London School of Hygiene and Tropical Medicine,
Keppel Street, London WC1E 7HT, England, G.B.
e-mail: g.clark at lshtm.ac.uk