dissenting scientists by showing that much of the genome of mitochondria had been transferred to the host cell’s
nucleus, preventing the mitochondria from being able to live on their own.[10] [11]
Wallin’s ideas regarding the endosymbiotic hypothesis were largely ignored for the next 50 years because scientists
were unaware that these organelles contained their own DNA. However, with the discovery of mitochondrial and
chloroplast DNA in the 1960s, the endosymbiotic hypothesis was resurrected. Lynn Margulis (1938–2011), an
American geneticist, published her ideas regarding the endosymbiotic hypothesis of the origins of mitochondria and
chloroplasts in 1967.[12]In the decade leading up to her publication, advances in microscopy had allowed scientists to
differentiate prokaryotic cells from eukaryotic cells. In her publication, Margulis reviewed the literature and argued
that the eukaryotic organelles such as mitochondria and chloroplasts are of prokaryotic origin. She presented a
growing body of microscopic, genetic, molecular biology, fossil, and geological data to support her claims.
Again, this hypothesis was not initially popular, but mounting genetic evidence due to the advent of DNA sequencing
supported theendosymbiotic theory, which is now defined as the theory that mitochondria and chloroplasts arose
as a result of prokaryotic cells establishing a symbiotic relationship within a eukaryotic host. With Margulis’ initial
endosymbiotic theory gaining wide acceptance (Figure 3.7), she expanded on the theory in her 1981 bookSymbiosis
in Cell Evolution. In it, she explains how endosymbiosis is a major driving factor in the evolution of organisms. More
recent genetic sequencing and phylogenetic analysis show that mitochondrial DNA and chloroplast DNA are highly
related to their bacterial counterparts, both in DNA sequence and chromosome structure. However, mitochondrial
DNA and chloroplast DNA are reduced compared with nuclear DNA because many of the genes have moved from
the organelles into the host cell’s nucleus. Additionally, mitochondrial and chloroplast ribosomes are structurally
similar to bacterial ribosomes, rather than to the eukaryotic ribosomes of their hosts. Last, the binary fission of these
organelles strongly resembles the binary fission of bacteria, as compared with mitosis performed by eukaryotic cells.
Since Margulis’ original proposal, scientists have observed several examples of bacterial endosymbionts in modern-
day eukaryotic cells. Examples include the endosymbiotic bacteria found within the guts of certain insects, such as
cockroaches,[13]and photosynthetic bacteria-like organelles found in protists.[14]
- T. Embley, W. Martin. “Eukaryotic Evolution, Changes, and Challenges.”NatureVol. 440 (2006):623–630.
- O.G. Berg, C.G. Kurland. “Why Mitochondrial Genes Are Most Often Found in Nuclei.”Molecular Biology and Evolution17 no. 6
(2000):951–961. - L.Sagan. “On the Origin of Mitosing Cells.” Journal of Theoretical Biology 14 no. 3 (1967):225–274.
- A.E. Douglas. “The Microbial Dimension in Insect Nutritional Ecology.”Functional Ecology23 (2009):38–47.
- J.M. Jaynes, L.P. Vernon. “The Cyanelle ofCyanophora paradoxa: Almost a Cyanobacterial Chloroplast.”Trends in Biochemical
Sciences7 no. 1 (1982):22–24.
Chapter 3 | The Cell 87