Encyclopedia of Evolution.pdf - Online Reading Center

0 symbiogenesis
like ciliated or flagellated cells; one must look closely, and
watch the process of their formation, to realize that they are
not cilia at all, but bacteria! In other words, spirochetes are,
today, doing the very thing that Margulis claimed they did
back when undulipodia came into existence.
One piece of evidence that is missing here, and which
was so important in gaining the acceptance of the symbiogenetic
origins of chloroplasts and mitochondria, is DNA.
It has not been established beyond doubt that undulipodia
(in particular, the structures from which they grow) and the
spindle apparatus (in particular, the structures from which
they grow) contain any DNA. Some studies have shown that
there may be DNA in these structures. If the DNA is present,
the conclusion that undulipodia and spindles have a
symbiogenetic origin will be hard to resist. The absence of
DNA will not, however, disprove the theory, because it is
possible that all of the DNA that the ancestral spirochetes
once possessed may have moved to the nuclei of the host
cells. In the absence of DNA evidence, Margulis and others
are seeking to determine whether the proteins found in spirochetes
and the proteins found in undulipodia and spindles
have structural similarities that are too great to be explained
by chance.
Another structure that may have had a symbiogenetic
origin is the hydrogenosome, a structure that produces hydrogen
and is found in some protist cells. In some cases the genes
that control hydrogenosome activity are all together in the
host cell nucleus and resemble bacterial rather than eukaryotic
genes, as if they were transposed to the nucleus from the
ancestral hydrogenosome, which may have been a mitochondrion
or a bacterium.
Margulis reconstructs the history of eukaryotic cells in
four stages, which she calls Serial Endosymbiotic Theory:
1. The nucleus had a bacterial origin. The most likely ancestor
for the nucleus is an archaebacterium (see archaebacteria)
similar to modern Thermoplasma. Some
archaebacteria have proteins that resemble the histones
associated with the DNA in eukaryotic chromosomes. The
ancestral cell was now a eukaryote, with a nucleus.
2. Spirochetes associated with this cell and eventually became
undulipodia and the spindle.
3. Aerobic bacteria moved into some of these cells and eventually
became mitochondria. Some of the protists that had
mitochondria became the ancestors of animals, fungi, and
plants.
4. Photosynthetic cyanobacteria were consumed, but not
digested, by some cells that already had mitochondria, and
eventually became chloroplasts. Some of the protists that
had chloroplasts became the ancestors of plants.
One of the strongest proofs of symbiogenesis is the fact
that the structures now called chloroplasts have originated
more than once. Most chloroplasts, like those of green algae
and of all terrestrial plants, resemble bacteria and are surrounded
by a simple membrane. But the chloroplasts of some
protists, such as dinoflagellates, have multiple membranes.
This is also true of the apicoplast of the Plasmodium falci-
parum protist that causes malaria, which is a degenerate, multiple-membraned
chloroplast. The Plasmodium does not use
its apicoplasts for photosynthesis. The multiple membranes
of these chloroplasts and apicoplasts suggest that they evolved
when photosynthetic eukaryotic cells invaded other eukaryotic
cells, stayed there, and simplified. In other words, regular
chloroplasts are degenerate cells inside a cell; dinoflagellate
chloroplasts and plasmodial apicoplasts represent degenerate
cells inside of degenerate cells inside of a cell! There is even
microscopic evidence that these chloroplasts have nucleomorphs,
which appear to be little, vestigial nuclei (see vestigial
characteristics).
A dramatic experimental demonstration of symbiogenesis
(albeit unintended) occurred when biologist Kwang Jeon
of the University of Tennessee at Knoxville noticed that his
cultures of the protist Amoeba proteus looked sick. Under
the microscope he could see that each amoeba was infected
with thousands of rod-shaped bacteria. The amoebae usually
digested bacteria, but these bacteria could resist the digestive
enzymes of the amoebae. Most of the amoebae died from the
bacterial infection. However, a few amoebae could resist the
bacteria and survive. Jeon cultured the survivors over and
over. He was artificially selecting for the amoebae that could
best resist the bacterial infection, as well as for the bacteria
that had the mildest effects on the amoebae. After about a
year and a half, the amoebae and bacteria had developed a
mutually beneficial relationship. After several years, the
coevolution of amoebae and bacteria had progressed so far
that neither of the species could survive without molecules
supplied by the other. They had become an essential part of
each other’s environment. Might the amoebae now be considered
a new species of protist? Might the bacteria now be
considered a new species as well? Perhaps they could even be
considered one new species of organism?
Another example of a symbiogenetic process in action
involves the bacterium Buchnera that infects some insects
such as aphids. Like an organelle, the Buchnera bacterium
can be transmitted from one generation to the next through
eggs. Because Buchnera manufactures certain amino acids
that aphids cannot get from the sap that they eat, it can be
considered mutualistic. Did mitochondria begin their evolution
in a similar way?
Evolutionary biologists Noriko Okamoto and Isao
Inouye have found evidence of a secondary symbiosis that
appears to be in progress. There is a green algal symbiont
that lives in certain protist host cells. The symbiont still has a
nucleus, mitochondria, and a large green plastid, but the flagella,
cytoskeleton, and internal membranes are gone. When
the symbiont divides, it produces two cells, one with the plastid
and one without. The cell without the green plastid then
develops a feeding apparatus and engulfs the cell with the
green plastid. This may represent a direct observation of a
“missing link” in the evolution of chloroplasts by symbiosis.
Recent experimental evidence has reconstructed some
other possible symbiogenetic pathways. Evolutionary biologist
Joel Sachs raised two kinds of bacteriophage (viruses
that live inside of bacteria) in laboratory bacteria. He used
antibiotics to eliminate any bacteria that did not contain both