Endosymbiotic hypothesis

The Endosymbiotic Hypothesis is a hypothesis about the origins of mitochondria and chloroplasts, which are organelles of eukaryotic cells. According to this, these originated as prokaryotic endosymbionts, which came to live inside eukaryotic cells. The hypothesis postulates that the mitochondria evolved from aerobic bacteria (probably proteobacteria, related to the rickettsias), that the chloroplast evolved from endosymbiotic cyanobacteria (autotrophic prokaryotes). The evidence for this theory is compelling as a whole, and it is now generally accepted.

The idea that the eukaryotic cell is a group of microorganism was first suggested in the 1920s by the American biologist Ivan Wallin. The endosymbiont theory of mitochondria and chloroplasts was proposed by Lynn Margulis of the University of Massachusetts Amherst. In 1981, Margulis published Symbiosis in Cell Evolution in which she proposed that the eukaryotic cells originated as communities of interacting entities that joined together in a specific order. The procaryote elements could have enterered a host cell, perhaps as a indigested prey or as a parasite. Over time, the elements and the host could have developped a mutually beneficial interaction, later evolving in an obligatory symbiosis.

Dr. Margulis has also proposed that eukaryotic flagella and cilia may have arisen from endosymbiotic spirochetes, but these organelles do not contain DNA and do not show any ultrastructural similarities to any prokaryotes, and as a result this idea does not have wide support. Margulis claims that symbiosic relationships are a major driving force behind evolution. According to Margulis and Sagan (1996), "Life did not take over the globe by combat, but by networking" (i.e., by cooperation, interaction, and mutual dependence between living organisms). She considers Darwin notion of evolution driven by competition to be incomplete.

Evidence for the theory

Evidence that mitochondria and chloroplasts arose via an ancient endosymbiosis of a bacteria is as follows:

• Both mitochondria and chloroplasts contain DNA, which is fairly different from that of the cell nucleus, in a quantity similar to that of bacteria. Further, they are surrounded by two or more membranes, and the innermost of these shows differences in composition compared to the other membranes in the cell. This is consistent with a cellular origin.
• New mitochondria and chloroplasts are formed only through a process similar to binary fission[?]. In some algae, such as Euglena, the chloroplasts can be destroyed by certain chemicals or prolonged absence of light without otherwise affecting the cell. In such a case, the chloroplasts will not regenerate.
• Much of the internal structure and biochemistry of chloroplasts, for instance the presence of thylakoids and particular chlorophylls, is very similar to that of cyanobacteria. Phylogenies built with bacteria, chloroplasts and eukaryotic genomes also suggest that chloroplasts are most closely related to cyanobacteria.
• DNA sequence analysis and phylogeny suggests that nuclear DNA contains genes that probably came from the chloroplast.
• Some genes encoded in the nucleus are transported to the organelle, and both mitochondria and chloroplasts have unusually small genomes compared to other organisms. This is consistent with an increased dependence on the eukaryotic host after forming an endosymbiosis.
• Chloroplasts appear in very different groups of protists, which are in general more closely related to forms lacking them than to each other. This suggests that if chloroplasts originated as part of the cell, they did so multiple times, in which case their close similarity to each other is difficult to explain.

References

• General textbook: Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts and Peter Walter, Molecular Biology of the Cell, Garland Science, New York, 2002. ISBN #0-8153-3218-1.
• Discusses theories on how mitochondria and chloroplast genes are transferred into the nucleus, and also what steps a gene needs to go through in order to complete this process: Jeffrey L. Blanchard and Michael Lynch (2000), Organellar genes: why do they end up in the nucleus?, TRENDS in Genetics, 16 (7), pp. 315-320. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10858662&dopt=Abstract
• Recounts evidence that chloroplast-encoded proteins affect transcription of nuclear genes, as opposed to the more well-documented cases of nuclear-encoded proteins that affect mitochondria or chloroplasts. Paul Jarvis (2001), Intracellular signalling: The chloroplast talks!, Current Biology, 11 (8), pp. R307-R310. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11369220&dopt=Abstract

Related Articles Evolution of flagella (Discusses the endosymbiont theory of the evolution of flagella, and has more on Margulis)