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Cells were complex even before mitochondria?

mitochondria in mammal lung tissue/Louisa Howard

From ScienceDaily:

Up until now, a number of theories have sought to explain how cells came to acquire mitochondria [power plants]. Although there is consensus as to the “how” ?the first mitochondria must have been a bacterium that entered another, and remained there, becoming part of the cell? the “when” has so far been unclear. Some scientists advocated an early incorporation of mitochondria, and considered that step as the first necessary to begin advancing toward eukaryotic cells as they are known today. Other theories proposed a later inclusion of mitochondria, as a more complex host cell could favor the entry of another cell and that cell’s permanence within its interior. Now, predoctoral scientist Alexandros Pitis and ICREA research professor and group leader at CRG Toni Gabaldón have clarified the matter, proposing a theory that would define the time frame for the acquisition.

The scientists used a diverse set of measurements to date the incorporation of several proteins into the eukaryotic lineage. They found that the arrival of proteins had come in a number of “waves,” and that those related with ancestral mitochondria matched those of the latest wave. “Our work demonstrates that the acquisition of mitochondria occurred late in cell evolution, and that the host cell already had a certain degree of complexity,” states Alexandros Pitis, lead author of the study. “Our study makes it possible to break down the steps of what is considered the greatest evolutionary leap after the origin of life. Understanding how complexity originated and evolved is important to better understand the mechanisms that govern cells, and by extension, the functioning of all living organisms,” concludes Gabaldón. More.

See also: Are there too many mitochondrial genome papers? (It’s a hot field just now.)

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Here’s the abstract:

The origin of eukaryotes stands as a major conundrum in biology1. Current evidence indicates that the last eukaryotic common ancestor already possessed many eukaryotic hallmarks, including a complex subcellular organization1, 2, 3. In addition, the lack of evolutionary intermediates challenges the elucidation of the relative order of emergence of eukaryotic traits. Mitochondria are ubiquitous organelles derived from an alphaproteobacterial endosymbiont4. Different hypotheses disagree on whether mitochondria were acquired early or late during eukaryogenesis5. Similarly, the nature and complexity of the receiving host are debated, with models ranging from a simple prokaryotic host to an already complex proto-eukaryote1, 3, 6, 7. Most competing scenarios can be roughly grouped into either mito-early, which consider the driving force of eukaryogenesis to be mitochondrial endosymbiosis into a simple host, or mito-late, which postulate that a significant complexity predated mitochondrial endosymbiosis3. Here we provide evidence for late mitochondrial endosymbiosis. We use phylogenomics to directly test whether proto-mitochondrial proteins were acquired earlier or later than other proteins of the last eukaryotic common ancestor. We find that last eukaryotic common ancestor protein families of alphaproteobacterial ancestry and of mitochondrial localization show the shortest phylogenetic distances to their closest prokaryotic relatives, compared with proteins of different prokaryotic origin or cellular localization. Altogether, our results shed new light on a long-standing question and provide compelling support for the late acquisition of mitochondria into a host that already had a proteome of chimaeric phylogenetic origin. We argue that mitochondrial endosymbiosis was one of the ultimate steps in eukaryogenesis and that it provided the definitive selective advantage to mitochondria-bearing eukaryotes over less complex forms. (paywall) – Alexandros A. Pittis, Toni Gabaldón. Late acquisition of mitochondria by a host with chimaeric prokaryotic ancestry. Nature, 2016; DOI: 10.1038/nature16941


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