The Endosymbiotic Theory is no different



The theory was first articulated by the Russian botanist Konstantin Mereschkowski in 1905. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts. Mitochondria developed from proteobacteria (in particular, Rickettsiales or close relatives) and chloroplasts from cyanobacteria. It suggests that multiple forms of bacteria entered into symbiotic relationship to form the eukaryotic cell. The horizontal transfer of genetic material between bacteria promotes such symbiotic relationships, and thus many separate organisms may have contributed to building what has been recognised as the Last Universal Common Ancestor (LUCA) of modern organisms.

Endosymbiosis – The Appearance of the Eukaryotes

Which evidence is consistent with the endosymbiotic hypothesis for the origin of the mitochondrion

Endosymbiotic Theory Introduction

In 1967 she proposed a contentious new hypothesis whichbecame her most important scientific contribution as the endosymbiotic theoryof the origin of mitochondria as separate organisms that long ago entered a symbioticrelationship with eukaryotic cells through endosymbiosis.

"She is best known for her theory of symbiogenesis, which challenges a centraltenet of neodarwinism.

Endosymbiosis: Lynn Margulis - Understanding Evolution

All evolutionary theories must offer an explanation in mechanistic terms of how it should or could have happened in order to be tested. The difficult thing with the endosymbiotic theory is that it proposes no real mechanism and most textbooks show the simplistic picture of a cell that swallows another cell that becomes a mitochondrion. Unfortunately, it is not so simple as that. There is a difference between the process of endosymbiosis and its incorporation in the germline, necessitating genetic changes. What were those changes? What was the host? Was it a fusion, was it engulfment, how did the mitochondrion get its second membrane, how did two genomes in one cell integrate and coordinate? The theory is also strongly teleological, illustrated by the widely used term ‘enslavement’. But how do you enslave another cell, how do you replace its proteins and genes without affecting existing functions? The existence of obligate bacterial endosymbionts in some present eukaryotes is often presented as a substitute for a mechanism, but they remain bacteria and give not rise to new organelles. So, before we can speak of the endosymbiotic as a testable scientific theory, we need a mechanistic scenario which is lacking at the moment.

Question What is the endosymbiotic hypothesis regarding the origin of mitochondria? What molecular facts support this hypothesis? To …
10/11/2017 · On Mar 3, 2014 Michael W Gray published: The Pre-Endosymbiont Hypothesis: A New Perspective on the Origin and Evolution of Mitochondria

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There are alternative explanations for the origin of mitochondria that can compete with the endosymbiotic theory and that are in line with the phylogenetic data. In an autogenic origin, mitochondria are evolutionary derived from the eukaryotic endomembrane system. Its evolution can be driven by the advantages to sequester metabolic activity in specialized compartments. As extensions to existing functionality, the targeting and import mechanisms could be developed gradually and based on existing eukaryotic functions, for instance already existing ER or organelle targeting. The acquisition of DNA would enable the metabolic vesicles to become relatively independent by expressing proteins that cannot be imported through normal organelle targeting or that facilitate mitochondrial functions. The mitochondrial genes could be derived from transposable elements, plastids or viruses and could come from either the nuclear genome or a bacterial genome. The components of the essential ATP-generating cascades suggest a gradual evolution based on expanding on an existing proton motive force coupled to ATP generation. Intermediates exist in the form of hydrogenosomes and mitosomes from amitochondriate primitive eukaryotes. Thus, we do have an alternative hypothesis that defines a gradual mechanism that includes intermediates fitness advantages for these steps, in line with Darwinian theory.

22/02/2016 · What are the proofs and counterarguments on endosymbiotic hypothesis of mitochondria's and chloroplasts's origin?

Plant Evolution Website - University of Waikato

It is also claimed that phylogenetic evidence firmly supports the endosymbiotic theory, for instance by showing relatedness between mitochondria and specific groups of bacteria. Some mitochondrial proteins show indeed a similarity with specific bacterial proteins, both at the structural and at the gene level. However, this does not show that mitochondria are derived from bacteria, but only that these proteins have a common descent. Next to an origin from bacteria, it can also be that these related proteins descended from a common ancestor, that only the genes were transferred or that bacteria picked up the genes from either the nucleus or the mitochondria. In general, large mitochondrial genomes contain a mix of presumed eukaryotic, archaeal and eubacterial genes, in contrast to what you expect if mitochondria are of eubacterial origin. Furthermore, a priori assumptions in phylogenetic analyses, such as long-branch attraction can bias phylogenetic tree analysis. This is illustrated by the amitochondriate organisms that can be either placed ancestral in the eukaryotic tree or derived, depending on whether you assume they once possessed mitochondria or not. Thus, as long as we do not have a clear picture of the last common ancestor and its relationship with eukaryotes, it will be difficult to interpret gene similarity as evidence for the endosymbiotic theory.

04/01/2018 · In the late 1960s Margulis (left) studied the structure of cells

Project Aquarius and the Story of Dr. Dan Burisch