Abiogenesis & Evolution


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Horizontal Gene Transfer in Prokayotes

Horizontal gene transfer - gene swapping - has blurred the evolutionary relationships (phylogeny) of prokaryotes (image), and continues to provide a mechanism for the sharing of antibiotic resistance between bacteria.
(see The net of life: Reconstructing the microbial phylogenetic networkV. Kunin, L. Goldovsky, N. Darzentas, and C. A. OuzounisGenome Res. 1 July 2005. pdf)

Three mechanisms of horizontal (lateral) gene transfer are recognized: direct bacterial conjugation, bacteriophage mediated transduction between bacteria, and bacterial transformation by uptake of DNA fragments.

A major form of vertical gene transfer followed serial endosymbiotic events, in which ingested purple bacteria and Cyanobacteria became eukaryotic mitochondria and chloroplasts respectively. The ingested prokaryotes are believed to have relinquished certain genes to the nuclei of their host cells, a process known as endosymbiotic gene transfer.

The Woodstock of Evolution -- The World Summit on Evolution (ScientificAmerican.com): Margaret Riley is a colleague of Lynn Margulis at the University of Massachusetts-Amherst. She provided the commentary on Gogarten's presentation (Tree of Life or Fuzzy Bush of Life), suggesting that we need a modification of Ernst Mayr's definition of a species to accommodate microbes. Ernst Mayr defined a species as: 'A group of actually or potentially interbreeding natural populations reproductively isolated from other such populations.'

The problem with applying this definition to microbes is that separate species are not truly reproductively isolated, and yet they still retain distinct features that keep them phenotypically apart. 'Although horizontal gene transfer can and does occur, it does not obliterate the phenotypic groupings of organisms,' Riley concluded."

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Protists in Evolution, and Symbiogenesis

The Woodstock of Evolution -- The World Summit on Evolution (ScientificAmerican.com): "Australian botanist Geoff McFadden, from the University of Melbourne, lectured on 'Protists and Cellular Phenomena in Evolution.'

Darwin ignored protists, but Ernst Haeckel included them in his comprehensive tree of life. Constantin Mereschkowsky was the first to appreciate the significance of protists in early eukaryotic evolution. A.F.W. Schimper noted that chloroplasts in plant cells very much resembled cyanobacteria. The the ultimate theoretical model was provided by Lynn Margulis: the key step was the endosymbiosis of cyanobacteria within a phagotrophic eukaryotic host, a process she calls symbiogenesis. The symbiotic theory of mitochondrial origin is supported by the different nature of internal and external membranes in mitochondria. In primary endosymbiosis, 1,000 genes were acquired by the nucleus from an incorporated cyanobacteria. A second round of gene transfer involved the engulfment of another plastid-containing eukaryote in secondary endosymbiosis. See also, Endosymbiosis and The Origin of Eukaryotes Illustration Endosymbiosis in Evolution

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