What type of bacteria evolved into mitochondria?

  • #1
Ygggdrasil
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Main Question or Discussion Point

An important step in the evolution of plants, animals and other complex, multicellular forms of life was eukaryogenesis, the evolution of eukaryotes. Eukaryotes are one of the three major classifications of life (alongside single-celled bacteria and archaea), and are characterized by cellular compartmentalization, an extensive membrane network inside of the cells, and the presence of mitochondria. A lot of recent work has focused on refining the origins of eukaryotes, which are thought to have evolved from the fusion between an archaeon and a bacterium (which became the mitochondria). Recent work has narrowed down the origin of our archaeal ancestor to among a newly discovered group of archaea dubbed the Asgard superphylum.

Scientists have also been working toward pinning down the origin of our mitochondria. Because scientists had thought mitochondria evolved from a group of bacteria known as the alphaproteobacteria (specifically a bacterium closely related to the present day Rickettsiales group), a research team led by Thijs Ettema (the scientist who discovered the Asgard archaea described above) set out to collect a diverse set of alphaproteobacteria from across the world to narrow down the origins of mitochondria. Instead of finding a close relative of the mitochondria, however, they instead found something even more surprising:
Since the 1970s, when researchers turned up similarities between DNA in eukaryotes’ mitochondria and bacterial genomes, scientists have suspected that the organelles descended from symbionts that took up residence within larger cells. A diverse class of bacteria called Alphaproteobacteria soon emerged as a likely candidate for the evolutionary origins of mitochondria. But a new analysis, published today (April 25) in Nature, suggests that mitochondria are at best distant cousins to known alphaproteobacteria lineages, and not descendents as previously thought.

“We are still left hungry for the ancestor of mitochondria,” says Puri Lopez-Garcia, a biologist at the University of Paris-South who was not involved in the study.
https://www.the-scientist.com/?articles.view/articleNo/52429/title/Mitochondria-s-Bacterial-Origins-Upended/

A wider sampling of alphaproteobacteria and improved techniques for analyzing DNA sequences seemed to suggest that our previous hypotheses about the origin of mitochondria were wrong. Instead of residing within alphaproteobacteria, the ancestor of the mitochondria may have been part of a more distantly related group of bacteria that remains to be defined. Discovering the exact origins of the mitochondria (and even identifying extant relatives of that ancestor) will be an exciting challenge for the researchers going forward.

Citation to the study discussed:
Martijn et al. (2018) Deep mitochondrial origin outside the sampled alphaproteobacteria. Nature Published online 25 April 2018
http://dx.doi.org/10.1038/s41586-018-0059-5
 

Answers and Replies

  • #2
BillTre
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It times like this when I wish I had a subscription to Nature or still worked in academia where Nature is easily available. :cry:

I think the origin of life/origin of eukaryotes is one of the most interesting subjects in biology these days.
Nature seems to get a lot of these kinds of articles.
Our understanding of the deepest of evolution slowly being better refined by finding new sequences, and the ability to perform analyses on larger data sets (needing greater computer power). :smile:
 
  • #3
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Viruses are a similar mystery, who asked for them?
A virus is some DNA which has evolved to be able to reproduce by hijacking eukaryotes
 
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  • #4
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A virus is some DNA which has evolved to be able to reproduce by hijacking eukaryotes
That's getting into the grey area. What did they evolve from? They couldn't have survived without a complete tool set to start from, could they?
A wider sampling of alphaproteobacteria and improved techniques for analyzing DNA sequences seemed to suggest that our previous hypotheses about the origin of mitochondria were wrong. Instead of residing within alphaproteobacteria, the ancestor of the mitochondria may have been part of a more distantly related group of bacteria that remains to be defined.
It's not really rocket science for life to adapt to what is most easily attained. If something does it one specific way why isn't it enough for something else to gain the same ability simultaneously?
 
  • #5
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That's getting into the grey area. What did they evolve from? They couldn't have survived without a complete tool set to start from, could they?
I would hypothesize that they evolved from bacterial/plasmid conjugation. Bacterial transfer of DNA. Leading to encapsulized transfer. Leading to capsule protein attachment evolution.

Bacterial conjugation allows horizontal gene transfers. Plasmids are a frequent gene transfer vehicle. The initial virus was probably a plasmid that contained a gene for overdriving conjugation. And once you've hijacked the system, adding genes for DNA binding proteins that have conjugation-like attachment and transfer could be sequentially added.
 
  • #6
I'm not sure if there have been any new theories and discoveries, but I recall the theory of symbiogenesis. It suggests that mitochondria along with the organelles of eukaryotic cells are the result of endosymbiosis.
 
  • #7
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I'm not sure if there have been any new theories and discoveries, but I recall the theory of symbiogenesis. It suggests that mitochondria along with the organelles of eukaryotic cells are the result of endosymbiosis.
RIght. The question is if a bacteria became endosymbiotic, and evolved into the non-independent mitochondria, did any of that endosymbiotic ancestral bacteria survive as an independent bacteria? Is Rickettsia a descendant from the common ancestor that also became mitochondria? Or is there even an independent bacterial descendant?

The referred to article indicates that the ancestral bacteria to mitochindria is not so cleanly related to Rickettsia. It may ultimately be the case that those mitochondial ancestor bacteria are extinct, and their evolved descendants are also extinct. I think that is the most likely situation, based on the current information.
 
  • #8
That sounds logical, thanks for explaining that for me. I got a bit confused. :rolleyes:
 

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