The Origin of Mitochondria

Question

For a long time I've just accepted, because it is just what everyone told me, that mitochondria became organelles in the cell when they were "engulfed" by another cell which acted like it's host. This is the endosymbiotic hypothesis.

"The endosymbiotic hypothesis suggests mitochondria were originally prokaryotic cells, capable of implementing oxidative mechanisms that were not possible to eukaryotic cells; they became endosymbionts living inside the eukaryote."

However, recently I looked at the wiki entry on mitochondria and saw there that it lists two main hypothesis, endosymbiotic and autogenous.

"In the autogenous hypothesis, mitochondria were born by splitting off a portion of DNA from the nucleus of the eukaryotic cell at the time of divergence with the prokaryotes; this DNA portion would have been enclosed by membranes, which could not be crossed by proteins."

It also states that, given that mitochondria have some striking similarities to bacteria, the most parsimonious hypothesis for mitochondrial evolution is that of endosymbiosis. (This question here deals with some aspects of multiple-occurence/convergent endosymbiosis).

My questions are;

1. How strong is the evidence for either of these leading hypotheses (perhaps there is a review article)? Is there evidence that is overwhelmingly in favour of endosymbiosis?

2. Further, are there any other (plausible) hypotheses to explain the origins of mitochondria (with or without supporting evidence)?

Answer 1

This is one of the most intriguing questions of eukaryotic evolution. As far as I know and have read, the autogenous theory is not accepted. There are quite some reviews on this topic. Also there is a wonderful book by Nick Lane on mitochondria called Power Sex and Suicide. You would be interested to read it.

There are no sufficient evidences for the evolutionary transition from prokaryotes to eukaryotes which somewhat suggests that this was some kind of quantum jump. For example:

• There are no microfossil records for the evolutionary intermediates
• Almost all eukaryotic features including organelles, syngamy, nucleus etc emerged simultaneously

So to answer how it exactly happened is quite difficult.

As for the autogenous theory; it would have been impossible for the huge eukaryotic cell (the precursor of mitochondria) to meet its energy demands without an organelle like mitochondria i.e. a prokaryote as large as a eukayotic cell wouldn't survive. You may check this post.

There are eukaryotic cells that lack mitochondria (eg. Entamoeba histolytica ) or have one with reduced functionality. However, these are not intermediates in evolution but have lost the functionality in a retrograde manner.

There is one more evidence to support the endosymbiosis theory: There is this observation that organelles that are less numerous in a cell have retained more of their genome compared to those with the organelles that are surplus numbers (eg. plastids vs mitochondria). This is called the Limited Transfer Window Hypothesis which reasons that the organelle to nuclear gene translocation would have happened because of organellar injury and the likelihood of a cell tolerating one is higher if there are more number of organelles.

This article suggests an alternative view that predatory bacteria like Bdellovibrio could have settled in a prokaryotic host. There are other cases of bacterial endosymbiont in a larger bacteria (I need some time to mine up the reference. Read it a while back) but these are not the the ancestors of eukaryotes.