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Thread: Lynn Margulis: 50th anniversary of her paper on eukaryote origin by symbiosis

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    Lynn Margulis: 50th anniversary of her paper on eukaryote origin by symbiosis

    Journal of Theoretical Biology | The origin of mitosing cells: 50th anniversary of a classic paper by Lynn Sagan (Margulis) | ScienceDirect.com has several papers in honor of that event, notably

    Physiology, anaerobes, and the origin of mitosing cells 50 years on - ScienceDirect by William F. Martin

    Symbiosis in eukaryotic evolution - ScienceDirect by Purificación López-García, Laura Eme, David Moreira (PLG is a woman)

    That is, endosymbiosis, internal symbiosis. Here is how we discovered it. In 1905, Russian botanist Konstantin Mereschkowski proposed that chloroplasts originated from what were called blue-green algae, what are now called cyanobacteria or blue-green bacteria. Some years later, in the 1920's, Ivan Wallin proposed that mitochondria originated from bacteria.

    But this endosymbiosis hypothesis was not popular for a long time. Most biologists preferred to believe that these structures originated from partitioning the cell interiors -- "autogenous" origin.

    But in the 1960's, along came Lynn Alexander Sagan Margulis (1938 - 2011), as she might be called. I note her maiden name and the last names of two husbands. Yes, she was married to Carl Sagan for a while. She became convinced of this theory, and in 1967, she wrote a very detailed paper advocating it: On the origin of mitosing cells - ScienceDirect She noted symbioses between microorganisms as a hint as to what led to endosymbiosis, and she also noted the Earth's geological history, like the atmosphere having little or no oxygen before around 2 billion years ago. Her then-husband likely helped her out in that part. She proposed:
    • Mitochondria are descended from purple bacteria, nowadays called alpha-proteobacteria
    • Chloroplasts are descended from cyanobacteria, the result of some 20 endosymbiosis events
    • Flagella, cilia, centrioles, and basal bodies are descended from spirochetes, some helix-shaped bacteria

    The first of these was provoked by the increasing oxygen in the atmosphere around 2 billion years ago. The second was for the host cell to get a source of food. The third was for motility, for propelling the organism.

    Her revival of endosymbiosis was very controversial and not very widely accepted at first, but as genetic and genomic evidence accumulated, it became partially accepted. I say "partially", because though there is strong evidence in support of parts of her hypothesis, there is similarly strong evidence against some other parts of her hypothesis.

    Mitochondria are well-established as being descended from alpha-proteobacteria. Some of the closest ones are rickettsia bacteria, and those ones live inside the cells of their hosts.

    Chloroplasts are well-established as descended from cyanobacteria, but their endosymbiosis happened only once: "primary endosymbiosis". Later, however, some protists turned some one-celled eukaryotic algae into their chloroplasts: "secondary endosymbiosis" and even some "tertiary endosymbiosis". Integration of plastids with their hosts: Lessons learned from dinoflagellates | PNAS -- some of these protists seem to have acquired "chloroplasts" more than once.

    The eukaryote flagellum and related parts are *not* derived from spirochetes. In particular, there is no evidence of a vestigial spirochete genome, as there is for alpha-proteobacterium and cyanobacterium ones in mitochondria and chloroplasts.

    About the genetic and genomic evidence, many species' mitochondrial and chloroplast genomes have been sequenced, and also sequenced are host-nuclear genes that code for some of their proteins. Mitochondria and chloroplasts have lost many of their genes to their host cells' nuclei, a process that has gone farther in some species than in others.

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    Though the origins of mitochondria and chloroplasts are now firmly settled, the origin of the rest of the eukaryotic cell has plenty of problems.

    The first is a big gap between prokaryotes and eukaryotes. Eukaryotes have a much more complicated internal structure, including a rather complicated endomembrane system. They also have meiosis, a kind of cell division without any analogy among prokaryotes. Meiosis, in turn, is part of a cycle that also includes cell fusion, something else absent from prokaryotes. That cycle is:

    Haploid - cell fusion - diploid - meiosis - haploid

    Another one is the source of the genes. The information systems are most closely related to Archaea, one of the two domains of prokaryotes, and the metabolic systems are most closely to Eubacteria, the other one. That one contains both alpha-proteobacteria and cyanobacteria, but many of the putative ancestral eukaryote's eubacterial genes are not closely related to their alpha-proteobacterial counterparts.

    Still another one is cell membranes. Both Eubacteria and Archaea can make terpenes, some branched-chain hydrocarbons. But Archaea cell membranes have terpene membrane lipids, while Eubacteria ones have fatty-acid ones. Fatty acids contain straight-chain hydrocarbons, making them unlike terpenes. But eukaryotic cell membranes contain fatty-acid ones and not terpene ones.


    A further problem is that a seeming intermediate, anaerobic protists, turned out not to be an intermediate. Many of them have mitochondrion-related genes and many of them have mitochondrion-like structures, though without genomes or oxygen use. Instead, they often release hydrogen. It is not as energy-efficient as combining it with oxygen, but it's better than nothing.

    So the ancestor of all the present-day eukaryotes must have had mitochondria, thus adding to its Frankenstein sort of origin.

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    A common theory for the origin of the nuclear membrane and the rest of the endomembrane system is, as one might guess, the autogenous theory. But PLG's paper mentions an alternative: an archaeon became an endosymbiont of a eubacterium. This explains the informational genes, the nuclear membrane, and the cell membrane's fatty acids. But unlike for mitochondria and chloroplasts, it was the symbiont that ended up collecting all the genes.

    As to what might have provoked that symbiosis, there is an interesting sort of metabolic cooperation that some organisms do. Hydrogen-releasing ones have the problem that if the hydrogen does not depart fast enough, then this hydrogen will slow down release of further hydrogen. However, methanogens consume hydrogen and carbon dioxide and release water and methane. Thus helping the hydrogen releasers to continue. Origin from this sort of symbiosis is the "hydrogen hypothesis" of eukaryotic origins or eukaryogenesis.

    Meaning that the ancestral eukaryote had at least three ancestors: a nuclear one, a cytosol one, and a mitochondrial one.

    Somewhere along the line, it must have gotten the capability of engulfing and devouring other organisms. This capability, phagocytosis, is common among present-day protists, and is also done by immune-system cells over the animal kingdom. Biologist Eugene Koonin has proposed something that I don't quite recall, but something like all this organism eating provoking the evolution of the endomembrane system as a way of keeping the nucleus's genome from getting swamped by genes from the organism's meals.

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    As to the organism with the most closely-related informational genes, it has long been suspected to be some archaeon, but which one has been difficult to identify.

    In Carl Woese's three-domain hypothesis, eukaryotes are the sister group of the archaea, branching off before the archaea produced their present-day branches. However, sequencing of more genes and improved gene-sequence has placed the eukaryotes inside the Archaea family tree, though exactly where continues to be difficult to identify.

    The best hypothesis at the moment is that the eukaryotes branched off from inside the "Asgard group" of archaea that are found in ocean-floor sedments and the like, organisms that are currently only known from sequencing genes from environmental samples. None of them has been grown in the lab, at least not yet.

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    Forgive me for only skimming, but wasn't the anniversary technically back in December? Still pretty cool.

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    Quote Originally Posted by Underseer View Post
    Forgive me for only skimming, but wasn't the anniversary technically back in December? Still pretty cool.
    That is indeed correct about that anniversary issue. The original paper came out in March 1967, and that makes that anniversary issue about 9 months behind.

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