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Endosymbiosis; a theory that proposes the evolution of the eukaryotic cell or yet is it another biological theory that is complete garbage? This is what this essay has been investigating, whether the evidence gathered over the last fifty years is good enough to support the endosymbiotic theory and to change the views of millions of biologists who still do not agree with it. The theory was first postulated in 1905 by a Russian biologist, Konstantin Mereschkowsky but was more popularised in the 1960s by Lynn Margulis. According to this hypothesis, about two billion years ago, a prokaryotic organism consumed a chemo-organotrophic bacteria giving rise to the eukaryotic cell and its organelles such as the mitochondrian. Later on this eukaryotic cell then consumed a cyanobacterium, giving rise to the chloroplast (plastid) through a secondary endosymbiosis event. Despite it being one of the most controversial and laughable theories when first hypothesised, the endosymbiotic model is now immensely respected by many biologists and many aspects have been confirmed spectacularly by ongoing evidence including phylogenetic analysis, genome analysis and comparisons of organelles to prokaryotic organisms. With the theory never going to be a hundred percent proven the strong evidence gathered has made it one of the strongest contenders for the evolution of the eukaryotic cell.

Introduction

The three major domains of life including the archaea, bacteria (prokaryotes) and eukarya were first established by Carl Woese using ribosomal RNA (rRNA) for phylogenetic analysis. The original divergence of lineage gave rise to bacteria and archaea about 3.5 billion years ago (bya), but not eukaryotes, these diverged about billions of years later. However, major changes took place in the Earth’s biosphere as consequence of the diversification of microorganisms from bacteria and archaea, such as the generation of an oxygenated atmosphere. Bacteria took advantage of this oxygen and developed the ability to respire oxygen, with the oxidation of organic compounds they were able to obtain more energy leading to the development of larger cell populations including the eukaryotic cell. Eukaryotic organisms contain several interrelated characteristics that distinguish them between prokaryotes. Table 1 below, shows the unique differences between prokaryotic and eukaryotic organisms, despite there being vast differences between the two organisms, the endosymbiotic theory still states that there huge significance between the two [1] .

Characteristic

Eukaryotes

Prokaryotes

Membrane Enclosed Organelles

Yes

No

Ribosomes mass

80S

70S

Plasmids

Rarely

Yes

Cell Wall component

Cellulose (in plants), Chitin (fungi)

Peptidoglycan, muramic acid

Histone proteins

Yes

No

DNA Structure

Contained in the nucleus and forms structures called chromosomes.

Circular and covalently closed. It is naked and lies free in the cytoplasm.

Genes arranged in operons

No

Yes

Nucleus present

Yes

No

Table 1: The differences of certain characteristics of eukaryotic and prokaryotic organisms

This got scientists probing the origin of eukarya, asking how this third domain of life which including a membrane enclosed nucleus and organelles, emerged. Many hypotheses have been proposed on the origin of the eukaryotic cell, the most supported is the endosymbiotic theory. Symbiosis is defined as an association between two or more different species. This definition clearly excludes associations between individuals of the same species. A major event in which symbiosis supposedly played a beneficial role was in the evolution of the eukaryotic cell and its organelles (i.e. chloroplast, mitochondria) through associations involving different species of prokaryotic cells. In this manner, the eukaryotes acquired the metabolic machinery of cellular respiration and photosynthesis from the endosymbionts, an endosymbiont being any organism that lives within the body or cell of another organism [2] .

Figure 1 [3] – The different symbiotic associations – Organisms that are involved in this process can either benefit, be harmed or not be affected by this association. In parasitism one of the species benefit, the degree of harm to the host is very severe and it depends on the host for food e.g. fungi, viruses and worms. In commensalism, one of the species benefit while the other one remains unaffected, e.g. remora suckerfish attach to the shark to gain food. In mutualism both organisms benefit from the symbiosis event.

The endosymbiotic theory claims that the eukaryotic cell is not a single entity but rather a combination of different prokaryote cells that evolved to live together in a mutually beneficial way. The endosymbiosis theory is best explained in the origins of the organelles of the mitochondria and the chloroplast. It was thought about two billion years ago, in which only prokaryotes inhabited the Earth, successive symbiosis occurred between different prokaryotic species as figure 1 on the page below illustrates and explains.

Figure 2 [4] – Endosymbiotic theory – Chloroplasts and mitochondria may have descended from a small cyanobacteria and aerobic bacteria respectively that were engulfed by another larger prokaryote. A large prokaryotic organism engulfed a chemo-organotrophic organism, but did not digest it and it became the eukaryotic cell with the common organelle known as the mitochondria (A). This eukaryotic cell, then similarly took up a species of cyanobacteria by a similar engulfing event (B). The cyanobacteria evolved into a chloroplast, enabling the eukaryotic cell to become photosynthetic (C). This process also involves the extensive loss of DNA from the bacteria symbiont as well as transfer of genes to the nucleus of the eukaryotic host.

As stated above, the key step that the endosymbiotic theory supposedly played a role in was the evolution of the two key organelles, the chloroplast and the mitochondria. Both the organelles are very similar in terms in both structure and function, but most importantly both of these evolved in a very similar fashion. The mitochondria (figure 2) are the key players in aerobic respiration, they facilitate the process of oxidative phosphorylation in which the flow of electrons is energetically favourable enough to produce a gradient of protons, which is finally used to synthesise ATP – arguably one of the most important molecules ever to have existed on Earth.

Figure 3 [5] – Mitochondria structure – Mitochondria are surrounded by two membranes. The outer membrane is quite permeable and is composed of proteins and lipids, the inner membrane is rich in proteins and is less permeable to ions and small organic molecules. The cristae are a series of folded internal membrane that has attached the stalked particles for the influx of protons and the enzyme ATP synthase that together catalyze the synthesis of ATP via oxidative phosphorylation. The matrix possesses the enzymes needed for the link reaction and Krebs cycle, the reactions that proceed before oxidative phosphorylation.

The chloroplast is the site of photosynthesis in plants, one of the most important organelles in the plant behind the nucleus. In this two step process, light captured from the sun is used to excite electrons in chlorophyll molecules and produce a proton gradient just like of the mitochondrian to produce ATP (Photophosphorylation), and then carbon dioxide is fixed to produce glucose and other sugars which are vital for the plants survival (Calvin Cycle).

Figure 4 [6] – Chloroplast structure – Chloroplast are surrounded by a double membrane, a permeable outer membrane and a less permeable membrane like that of the mitochondria. The stroma is a fluid filled matrix which houses the Calvin Cycle and also contains storage structures such as starch grains. The grana are stacks of disc like structures called thylakoids which contain the chloroplast pigments. They are arranged in complexes called photosystems which is very important in capturing light and producing ATP.

The endosymbiotic theory is strongly supported by findings from structural, molecular and biochemical studies of the eukaryotic cell. Circumstantial evidence commonly used to support the theory of endosymbiosis is the similarity of the organelles of the mitochondria and chloroplast compared to the bacteria. However more rigorous evidence to support this theory comes from the examination of evolutionary relationships of the mitochondria and chloroplast genomes via phylogenetic analysis.

Kwang Jeon’s surprise discovery

In 1966, a discovery was made by microbiologist Kwang Jeon that supported the hypothesis of endosymbiosis; that bacteria can evolve into organelles. Jeon was studying a single cell protest species Amoeba proteus which is mostly aquatic living in fresh water ponds and moves by extending a finger of protoplasm called pseudopodium. But accidently one of his cultures containing the organism became contaminated and infected by a bacterium. Jeon was intrigued that not all of the infected amoebas died straight away as some continued to grow, so he maintained those infected cultures for further analysis. About five years later, the descendant amoebas were host to many bacterial cells, yet they were still healthy and alive. When those amoebas received antibiotics that usually do not harm amoeba, they died. This unintentional experiment confirmed that those amoebas had come to rely on the bacteria that had infected them originally. Through transplanting experimentation, Jeon found out that the nucleus of the amoebas could not live without the once pathogenic bacteria. Studies had showed the infected amoebas had lost the ability to make an essential enzyme and depended on the bacteria to make them. The bacterial cells had become vital endosymbionts and Jeon’s accidental discovery proved that it was possible for an organism to become dependent on and a functional part of invading organisms [7] . “Rather than eliminating competitors, evolution eliminated competition itself on the basis of symbiotic relationships [8] .

Symbiosis between present day photosynthetic organisms and non-photosynthetic organisms also support the endosymbiotic hypothesis of the formation of the eukaryote. For instance, Cyanophora paradoxa, a biflagellate protist, engulfed an endosymbiotic cyanobacterium (known as a cyanelle), which once taken up functions as a chloroplast and supplied the host cell with photosynthetically reduced carbon [9] . These discoveries basically proved that the theory of endosymbiosis does actually exist, and it could have led to the formation of the eukaryotic cells and its organelles.

Genetic Evidence

Supporting the symbiotic origin of the mitochondria and chloroplast is both the organelles possess their own genetic material, DNA which is distinctly different from the nuclear DNA of eukaryotic cells. The architecture of most the mitochondria and all of the chloroplast DNA is evidentially prokaryotic, being circular and having a single origin of replication, as figure 4 below clearly indicates. The genes of the mitochondria are also arranged in operons which are “one or more genes transcribed into a single mRNA and under the control of a single regulatory site [10] ” which also reflects prokaryotic ancestry, with similar arrangement of genes. Lactose uptake is controlled by an operon in prokaryotic organisms, with the binding of RNA polymerase blocked by a repressor protein, but is not in eukaryotic organisms. This organization of genes is with stark contrast compared to that of the eukaryotic nucleus which has multiple, linear chromosomes and individual genes, each with its own regulatory elements [11] .

Figure 5 [12] – Map of the human mitochondrial genome – The circular genome contains over 15,000 base pairs. The genome encodes the 16S and 12S rRNA, which correspond to the prokaryotic 23S and 16S rRNAs.

Furthermore, evidence suggests over evolutionary time, these organelles lost many genes that were unnecessary for life as an organelle inside the new host cell, and many genes were transferred to the nucleus (Lake and Rivera 1996, Martin 1996). Thus, non-phototrophic eukaryotic cells are genetic contain DNA from two different sources, the first from the endosymbiont and the second the host cell nucleus. Photosynthetic eukaryotic contain three sources of DNA, the mitochondrial and chloroplast endosymbionts and the nucleus. In the mitochondrian of a freshwater protozoan Reclinomona, it bears the largest collection of mitochondrial genes coding for 97 genes and contains 69,034 base pairs. However as table 2 (page 6) illustrates, the human mitochondria genome is less than a quarter of this size meaning the base pairs missing were almost certainly genes coding prokaryotic characteristics from its bacterial ancestors that were not required. According to Anderson and co-workers in 1998 many genes remaining in Reclinomonas mitochondria are strikingly similar to that of the obligate intracellular parasite, Rickettsia prowazekii the unicell causing typhoid disease [13] . Therefore this provides supportive evidence that some mitochondria in some present day eukaryotic organisms still contain genes that caused them to become a prokaryote, and hence shows their bacterial past just as the endosymbiotic theory proposes.