A Brief History of Mitochondria: The Elegant Origins of a Magnificent Organelle

Part III - Implausibility of the Autogenous Model

While we now know that the wealth of evidence for the endosymbiotic origins of mitochondria makes this theory virtually undeniable, such has obviously not always been the case. Recall that Margulis' publication of On the Origin of Mitosing Cells – which, we should again take note, was largely hypothetical – was met with marked intransigence – and this in 1967 (Lake, 2011)! At the time, the prevailing hypothesis was that all of the Eukarya arose from a single, ancestral prokaryote and that all membrane-bound organelles arose from the infolding of existing cytosolic elements and plasmid-mediated compartmentalization events which ultimately led to the intricately connected organellular systems we today observe in all modern eukaryotes. This idea is referred to as the autogenous hypothesis (The George Washington University, n.d.; Keeling, 2014). Initially, observational evidence did seem to suggest that mitochondria may have evolved autogenously. For instance, not all mitochondrial DNA is in fact circular; the mitochondrial chromosome of members of the protozoan genus Paramecium is linear (Pritchard & Cummings, 1981). Furthermore, most mitochondria exhibit a reticular morphology far removed from that of the prokaryote-like, bacillus-shaped drawings of mitochondria commonly found in textbooks (Davison & Garland, 1977, p. 233s; Hardin, et al., 2012, pp. 254-257). This school of thought, of course, has fallen greatly out of favor in view of the many evidentiary indications of the validity of the endosymbiotic model, as detailed in Part II of this case study. To quote evolutionary biochemist Nick Lane's superb paper Energetics and genetics across the prokaryote-eukaryote divide on the issue, "There is little doubt that all known eukaryotic cells share a common ancestor that arose only once in four billion years of evolution. Common traits range from the conserved position of many introns, to the structure of nuclear pore complexes, to complex traits such as syngamy [endnote 7] and two-step meiosis. It is implausible that all of these shared properties arose [largely] by lateral gene transfer (which is inherently asymmetric in mechanism) or convergent evolution (which implies that traits like intron position are dictated by selective constraints, rather than historical contingency). Common ancestry is [...] the most parsimonious explanation" [ennote 8] (2011). Common ancestry (and therefore endosymbiosis) is the most parsimonious explanation for the evolution of the modern mitochondrion, indeed.

Interestingly however, over the past several years, evidence is beginning to be brought to light that some eukaryotic cellular elements, namely the Golgi complex and endoplasmic reticulum (among other members of the endomembrane trafficking system), may have indeed originated in an autogenous manner! Read (in its entirety) the 2008 paper Phylogeny of endocytic components yields insights into the process of nonendosymbiotic organelle evolution by Dacks, et al. for a solid overview of some of this recent work.

ASSEMENT ITEMS

Now that you have had the opportunity to survey the emerging evidence for the likely autogenous evolutionary origin of some organelles such as the Golgi complex, compare the strength of the evidences detailed in the paper by Dacks, et al. for this likely scenario to those historically proposed in defense of an autogenous mitochondrial genesis by writing a brief, informal paper of no more than 700 words, laying out what specific evidence you think would need to be uncovered in order for the autogenous hypothesis to regain support as a valid explanation for the natural history of mitochondria.