Endosymbiosis - Origin of Mitochondria and Chloroplasts
One of the most fascinating concepts to gain popularity in recent times is the endosymbiotic theory for the origin of the eukaryotic cell
According to this theory:
€a prokaryotic cell capable of engulfing other prokaryotes, engulfed aerobic bacteria.
-Rather than digesting them, the bacteria remain, as symbionts, benefiting the host cell by removing harmful O2 and helping in the production of ATP.
- As interdependence between the aerobic bacterium and the host cell grows, the bacterium becomes the mitochondrion.
- Some of these cells also engulf and keep blue-green algal cells which become chloroplasts.
Endosymbioic origin of mitochondria and chloroplasts is an old idea:
The physical appearance of chloroplasts and mitochondria as observed by light microscopy was the justification used by Schimpler (1883) to make the first explicit proposal of symbiotic, bacterial origin of plastids, while Walin (1922) did the same for mitochondria.
These observations appeared to be supported later by electron microscopy when it was discovered that both organelles were surrounded by two membranes - the inner one supposedly belonging to the symbiont and the outer one a remnant of the membrane used by the host cell to engulf the symbiont.
Today, the endosymbiotic theory is most closely associated with the work of Lynn Margulis. She has further added to the hypothesis in two ways:
1. She has suggested that the eukaryotic flagellum arose from an ectosymbiotic spirochete bacterium. Confirmed evidence for this is rather slim: spirochetes are known to attach to the surface of a protozoan (Myxotricha paradoxa) that lives in the digestive tract of termites and to provide it with locomotion. (Spirochetes do not contain microtubules and flagella do not have their own strand of DNA as Margulis has claimed).
2. No prokaryote living today has ever been known to have another prokaryote living symbiotically inside of it. Is symbiosis in prokaryotes even possible? Margulis has therefore suggested that a prokaryote first developed a membrane around its DNA to form a "protoeukaryote" without anyother membrane-bound organelles. This protoeukaryote then is the organisms that serves as the host for the bacteria which eventually become mitochondria, chloroplasts and flagella.
The endosymbiotic origin of mitochondria and chloroplasts is widely believed because of the many similarities between prokaryotes and these organelles:
1. Mitochondria and chloroplasts are similar in size and shape to prokaryotes
2. Mitochondria and chloroplasts have their own DNA that lack histone proteins, that is circular, and is attached to the inner membrane as is the DNA of prokaryotes
3. Organellar ribosomes are more similar in size to prokaryotic ribosomes
4. Mitochondria and chloroplasts divide by fission, not mitosis.
5. Mitochondria arise from preexisting mitochondria; chloroplasts arise from preexisting chloroplasts (they are not manufactured through the direction of nuclear genes).
6. Outer membrane or chloroplasts or mitochondria would have been synthesized by the original "host" cell and used to engulf the endosymbiotic bacteria that became the mitochondria. The outer membrane has structural and chemical similarities to the eukaryote cell membrane.
Note: similar evidence to support the idea that spirochaete bacteria gave rise to flagella does not exist. Tubulin (the primary component of microtubules) has not been found in any prokaryote, and DNA has never been found in flagella. Most evolutionary biologists reject the idea that flagella originated by symbiosis.
Endosymbiosis is undoubtedly a fascinating concept and, at first glance, the evidence appears to support it as the mechanism for the evolution of chloroplasts and mitochondria. But it really isn't very good evidence - it is questionable on two counts.
1. It supports the alternative hypothesis equally well (in fact some of the features of mitochondria may be explained better by the alternative hypothesis).
2. It is the wrong sort of evidence because none of these data have been shown to be synapomorphies between prokaryotes and organelles.
Alternative Hypothesis - Autogenous
According to the autogenous model, the eukaryotes arose directly from a single prokaryote ancestor by compartmentalization of functions brought about by infoldings of the prokaryote plasma membrane. This model is usually accepted for the endoplasmic reticulum, golgi, and the nuclear membrane, and of organelles enclosed by a single membrane (such as lysosomes). According to the autogenous hypothesis, mitochondria and chloroplasts have evolved within the protoeukaryote cell by compartmentalizing plasmids (vesicles of DNA) within a pinched off invagination of the cell membrane.
Similarities between mitochondria or chloroplasts and eubacteria can be accounted for by mosaic evolution in which the components in the compartment evolve more slowly than other parts of the cell, and thus retain many eubacterial features. Mitochondria or chloroplasts may have acquired their double-membrane status by secondary invagination or more elaborate folding of membranes.
Other problems with the "Evidence" for endosymbiosis
1. Organelles are the same size and shape are bacteria - It is possible to find some chloroplasts the same size and shape as some bacteria, but the range in size and shape is so great we cannot rule out that they are similar just by chance.
Many mitochondria actually have a reticulated structure. Mitochondrial profiles seen in sections with the electron microscope only appear to be the same size and shape as bacteria but these are just the branches of a large reticular structure.
