Re: Endosymbiosis
Arthur V. Chadwick (chadwicka@swac.edu)
Sat, 24 Feb 1996 11:47:28 -0800>
> "Molecular evidence has now amply confirmed that mitochondria and
> plastids originated as bacterial symbionts. Molecular (particularly
> sequence) data have further pin-pointed those contemporary
> prokaryotes with which mitochondria and plastids are specifically
> affiliated. Two eukaryotic organelles have as their closest living
> relatives the eubacteria (Bacteria _sensu_), with plastids originating
> from within the blue-green algae (Cyanobacteria) and mitochrondria
> from within the alpha-subdivision of the purple bacteria (Proteobacteria).
> It is still unclear whether each organelle is the result of a single
> endosymbiosis of this type (monophyletic origin), or whether each
> arose more than once via prokaryote-eukaryote associations in
> difference eukaryotic lineages (polyphyletic origin). [...]"
> - Michael W. Gray - Origin and evolution of organelle genomes,
> p884, Current Opinion in Genetics and Development 1993 3:844-890.
>
>
> "The symbiotic origin of chloroplasts and mitochondria has been
> generally accepted for a decade or so [reference list deleted], as
> has the secondary symbiotic acquisition of the chloroplast by
> photosynthetic members of the kingdom Chromista [...]
> Acceptance of the symbiotic origins of mitochondria, chloroplasts
> and chromist perplastid membranes rests on three lines of argument:
> (a) a sufficiently close resemblance between the organelles and
> putative potential symbiotic ancestors to make the conversion of
> the symbiont to the organelle genetically and developmentally
> plausible; (b) the identification of a potential host that entirely
> lacks the organelle and its molecular precursors, but which would
> have a selective advantage in acquiring it by endosymbiosis;
> (c) phylogenetic evidence that clearly demonstrates that the
> different organelles actually had a difference ancestor from the
> nucleus and the rest of the cytoplasm. Peroxisomes satisfy the
> first two criteria but not the third, whereas cilia and nulcei
> satisfy none of them. Mitochondria, chloroplasts and the chromist
> satisfy all three to such a high degree that no reasonable biologist
> seriously doubts their symbiotic origin."
> - T. Cavalier-Smith - The number of symbiotic origins of organelles.
> pp. 91-21, in Biosystems 1992 28:91-106.
>
>The following references are useful starting points on the subject:
>MW Gray & WF Doolittle: Has the endosymbiont hypothesis been proven?
> (1982) Microbiol. Rev. 46:1-42
>MW Gray: The endosymbiont hypothesis revisited. In _Mitochondrial
> Genomes_; Int. Rev. Cytol. (1992) DR Wolstenholme & KW Jeon eds.
> 141:233-357
>
>Regards, Tim Ikeda (timi@mendel.berkeley.edu)
>
>
>
Art
http://chadwicka.swac.edu