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.

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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!