lpetrich
Contributor
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:
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.
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
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.
