-
Consolidation of the Eukaryotic Cell Endosymbiosis The
endosymbiotic theory was first articulated by the Russian botanist
Konstantin Mereschkowski in 1905. He was preceded in his work by
botanist Andreas Schimper, who had observed in 1883 that the
division of chloroplasts in green plants closely resembled that of
free-living cyanobacteria. Schimper had tentatively proposed (in a
footnote) that green plants had arisen from a symbiotic union of
two organisms.
Ivan Wallin extended the idea of an endosymbiotic origin to
mitochondria in the 1920s. These theories were initially dismissed.
However, a more detailed electron microscopic comparison between
cyanobacteria and chloroplasts by Hans Ris using the electron
microscope in the 1960s gave the theory new impetus. He became
interested in reports of cytoplasmic genes and DNA in cytoplasmic
organelles. He showed that chloroplasts resembled blue-green algae
in organization; the DNA-containing nucleoid and ribosome
organelles also were consistent with those of bacteria, while
mitochondria resembled bacteria. This revived an old hypothesis
that chloroplasts and mitochondria originated from endosymbiotic
microorganisms.
Symbiosis in Cell Evolution A graduate student in botany, Lynn
Margulis, pursued the theory with vigor and imagination, leading to
its general acceptance today. The endosymbiotic hypothesis was
fleshed out and popularized by her book in 1981 work Symbiosis in
Cell Evolution. In it she argued that eukaryotic cells originated
as communities of interacting entities.
“The advances of molecular biology, molecular genetics, electron
microscopy and other fields of modern biology suggest that
symbiosis was a major mechanism in the establishment of the first
eukaryotes from which the ancestral protoctists evolved (Margulis
et al.1990). We also know that members of the three kingdoms-fungi,
plants and animals – have protoctist ancestors. We know too that,
at least in the microcosm, genes cross taxonomic boundaries
rampantly, because DNA travels easily in the form of small
replicons: plasmids, viruses, transposons and so forth (sonea and
panisset 1983). Thus, many mechanisms beside random mutation cause
change in the hereditary endowments of organisms, including animals
and plants.”
The evidence for endosymbiosis of mitochondria, plastids and
ancient bacteria includes:
(1) Both mitochondria and plastids contain DNA similar to that
of bacteria (in being circular). They are quite different from
nuclear DNA. (2) Much of the internal structure and biochemistry of
plastids, i.e. chlorophylls, is very similar to that of
cyanobacteria.
-
(3) Some proteins encoded in the nucleus are transported to the
organelle, and both mitochondria and plastids have small genomes
compared to bacteria. This is consistent with an increased
dependence on the eukaryotic host after forming an endosymbiosis.
(4) Most genes on the genomes of organelles have been lost or moved
to the nucleus. Most genes needed for mitochondrial and plastid
function are located in the nucleus. Many originate from the
bacterial endosymbiont. (5) Plastids are present in very different
groups of protists, some of which are closely related to forms
lacking plastids (e.g., volvox vs. euglena and chloroplasts). (6)
Among the eukaryotes that acquired their plastids directly from
bacteria (known as primoplantae), the glaucophyte algae have
chloroplasts that strongly resemble cyanobacteria. In particular,
they have a peptidoglycan cell wall between their two membranes.
(7) These organelles' ribosomes are like those found in bacteria
(70s). Proteins of organelle origin, like those of bacteria, use
N-formylmethionine as the initiating amino acid.
Mitochondrial Life Cycle G. A. HORRIDGE of the Gatty Marine
Laboratory and Department of Natural History, the University of St.
Andrews, Fife, Scotland, presented The Giant Mitochondria of
Ctenophore Comb-Plates in the Quarterly Journal
MicroscopyScience1964:
The elongated cells bear the continually active giant cilia of
the combs. These contain numerous large mitochondria, up to 8 µ by
6 µ in size, which are filled with irregular tubular cristae. The
ciliated cells are up to 100 µ long, but only 10 µ wide, and from
their centrally situated nucleus can be traced through a succession
of stages, tentatively interpreted as the formation, growth,
erosion, and final dissolution of mitochondria.
Small ones occur near the nucleus in the region of the nuclear
membrane, which there may be irregular, puffy, and electron dense.
Some small mitochondria are surrounded by
amorphous material that stains heavily with lead; others lie
against the nuclear membrane, as if in intimate relation with
it.
Cristae of mitochondria, which are interpreted as juvenile, are
filled with an amorphous material and some of the cristae open to
the outside of the mitochondrion. Towards the ciliated end of the
cells the appearance of the mitochondria suggests that they are
breaking down; this is the region where food particles are eroded
and where the cilia consume energy. Here the mitochondria are
shrunken and around them are numerous vesicles; their cristae are
fewer and they open into the cytoplasm. Similar vesicles, which are
apparently of mitochondrial origin, are extruded between the cilia
from the cells. The proposed cycle of generation and
-
disintegration of mitochondria, based upon morphology, is so far
an unproved hypothesis.
Organization of Eukariocytes Organisms can generally inherit
genes in two ways: from parent to offspring (vertical gene
transfer), or by horizontal or lateral gene transfer, in which
genes jump between unrelated organisms. This is a common phenomenon
in prokaryotes. Lateral gene transfer has complicated the
determination of phylogenies of organisms since inconsistencies
have been reported depending on the gene chosen. Carl Woese came up
with the three-domain theory for the organization of life
(eubacteria, archaea and eukaryotes). This is based on his
discovery that the genes encoding ribosomal RNA are ancient and
distributed over all lineages of life with little or no lateral
gene transfer. Therefore various rRNA lines are commonly used as
molecular clocks for reconstructing phylogenies.
Protozoa, the animal-like protists Protozoa are mostly
single-celled, motile protists that most often feed by
phagocytosis, though there are many exceptions. They are usually
only 0.01-0.5 mm in size, generally too small to be seen without
magnification., Protozoa are divided into groups: •Flagellates such
as euglena, •Amoeboids; i.e. amoeba proteus, •ciliates; paramecium,
stylonychia •Sporozoa, non-motile parasites and paracytes.
Algae, the plant-like protists Algae include many single-celled
organisms that are also considered protozoa, such as Euglena, which
many believe have acquired chloroplasts through secondary
endosymbiosis. Others are non-motile, and some (called seaweeds)
are truly multi-cellular, including the following groups: (1)
chlorophytes -- the green algae related to higher plants (2)
rhodophytes or red algae,
(3) heterokontophytes -- brown algae, diatoms
-
(4)The Green and Red Algae The green and red algae, along with a
small group called the glaucophytes, appear to be close relatives
of other plants, and so some authors treat them as plantae despite
their simple organization. Most other types of algae, however,
developed separately. They include the haptophytes, cryptomonads,
dinoflagellates, euglenids, and chlorarachniophytes, all of which
have also been considered protozoans. Note some protozoa host
endosymbiotic algae, as in Paramecium bursaria or radiolarians,
which provide them with energy but are not integrated into the
cell.
Fungus-like Protists Various organisms with a protist-level
organization were originally treated as fungi, because they produce
sporangia. These include chytrids, water molds, and
labyrinthulomycetes. Of these, the chytrids are now known to be
related to other fungi and are usually classified with them. The
others are now placed among the heterokonts (which have cellulose
rather than chitin walls) and the Amoebozoa (which do not have cell
walls).
Biodiversity As example of diversity of the protists, this
survey was presented by N. D. Lavine College of Veterinary
Medicine, University of Illinois, Urbana 61801, 1987, “The number
of species of rodent coccidia and of other protozoa”: “About 447
species of coccidia have been named from the 1687 living, known
species of rodents; 207 host species, 92 host genera, and 15 host
families are represented; this is about 12% of the known species of
rodents. About 4600 species of apicomplexan protozoa have been
named. Assuming that the same proportion of the total number of
apicomplexan species has been named as of the coccidian species,
there must actually be about 38,333 species of apicomplexan
protozoa. There are 5.4 times as many protozoan genera as of
apicomplexan genera. Assuming that the number of species in each
genus is the same for all the protozoa as it is for the
-
apicomplexa, there may actually be 206,998 species of protozoa.
This may be too conservative an estimate. Based on other criteria,
an estimate of over 20 million species could be made.
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
Of this vast group of different species with all variations of
organelles, only a few reach independence => fewer reach
multi-cellularity => fewer progress to differentiation of cell
functions => then organ systems => nervous system =>
defense mechanisms => survival instincts => intelligence. Our
Humanity is ultimately a product of this steadily increasing
organization. iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
Genetic Variation in Mitochondria Michael W. Gray, - B. Franz
Lang, and - Gertraud Burger- presented an analysis of the genetics
of mitochondria in the Annual Review of Genetics, Vol. 38: 477-524
(Volume publication date December 2004) MITOCHONDRIA OF PROTISTS ▪
Abstract: “Over the past several decades, our knowledge of the
origin and evolution of mitochondria has been greatly advanced by
determination of complete mitochondrial genome sequences. Among the
most informative mitochondrial genomes have been those of protists
(primarily unicellular eukaryotes), some of which harbor the most
gene-rich and most eubacteria-like mitochondrial DNAs (mtDNAs)
known. Comparison of mtDNA sequence data has provided insights into
the radically diverse trends in mitochondrial genome evolution
exhibited by different phylogenetically coherent groupings of
eukaryotes, and has allowed us to pinpoint specific protist
relatives of the multicellular eukaryotic lineages (animals,
plants, and fungi). This comparative genomics approach has also
revealed unique and fascinating aspects of mitochondrial gene
expression, highlighting the mitochondrion as an evolutionary
playground par excellence.”
Marriage of Two Worlds To create the more modern eukaryotic
cell, several important factors had to come together. Eukaryotic
cells have many important improvements over the pre-existing
prokaryotic cell. They have new improved membranes, improved
genetics, and greatly increased efficiency and energy production.
They incorporated mitochondria --but for oxygen protection at
first, not energy. To achieve the improvement of the new cell type,
all the needed materials had to be present. Each mutation, leading
to increased organization, had to be useful in and of itself. The
unsaturated fats and ring structures required for the improved
membranes were present because they absorb toxic free radicals. At
high enough levels, these lipid chain
-
and ring structures self assemble into membranes in a water
environment. At first, these membranes would form a disorganized
protective cocoon wall, placing a barrier of anti-oxidant
protection between the organism and the outside world.
Cyanobacteria were the first known fossils. They were the carriers
of the RNA world. Ribosomes of the RNA world have the ability to
carry information code. As Tom Cech proved, they could also
reproduce themselves. Their protein enzymes could be produced with
veracity and in much greater quantities. This had obvious
advantages in the reproduction race and the new lipid membrane
cocoons served to protect them from ultraviolet radiation and free
radicals.
Mitochondria may have evolved in the fumerols of the deep ocean,
from sulfur metabolism organisms. This environment provides the
high acid gradients that free-living mitochondria might have lived
on directly. Then, as oxygen was added to the environment, the
mitochondria were able to survive and offer protection for
surrounding organisms by using up toxic oxygen.
The final marriage would be achieved if the host cell produced
thiamine and a polyunsaturated cocoon. The presence of thiamin
would develop the “thiamin pump phenomenon,” which creates a H+
gradient that would attract mitochondria. This
-
occurs spontaneously. The inside of the cell becomes alkaline
and the area outside the cell in the polyunsaturated cocoon would
be acidic, which is perfect for the mitochondria. The mitochondria
would feed on the lipids and the lipid radicals that are formed.
The polyunsaturated lipids act as scavengers of radicals and bring
them to the mitochondria. Both organisms have the advantage of the
other’s free radical protection.
Soon, from an evolutionary stand point, the membrane would
solidify with the mitochondria inside. The prokaryotic RNA organism
would form the nucleus and the polyunsaturated cocoon would become
the cytosol surrounded by the cell membrane. The nucleus and its
genetic material are protected by the cocoon and by respiratory
oxidation to eliminate O2, provided by the mitochondria.
Mitochondria would be free living within the new eukaryotic cell,
protecting it from oxygen while providing energy from the Krebs
cycle in the form of ATP. Perfect Symbiosis!