Endosymbiosis: critiques on my essay

[FreeThinker]

Hello all,

 

 

I recently wrote an essay on endosymbiosis. I have received feedback from the teachers, but it wasn’t very satisfactory in my opinion. So I figured that if any of you guys could give me some constructive feedback it would only help me improve .Any feedback will be greatly appreciated!

 

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Endosymbiosis provides an explanation for the origins of DNA-containing organelles. Critically evaluate the evidence for and against this theory.

 

 

Introduction

It is rather ironic that Darwin’s now famous book “On the Origins of Species” never dealt with how species originate. To explain natural phenomena such as the origins of Eukaryotic cells, Darwin’s explanation of a gradual accumulation of beneficial mutations might not be the whole story; endosymbiosis is the theory that attempts to fill the gap. Symbiosis is used to refer to the relationship between a host (animal or plant) and the smaller organisms (the symbiont). The term “endosymbiosis“ refers to the relationship where a symbiont lives within the host. Endosymbiosis provides an explanation for the origins of plastids and mitochondria, proposing that they were once free living bacteria that merged with a host organism to form Eukaryotic cell (C. Mereschkowsky, 1905), (Margulis 1970). This essay will critically evaluate the evidence in support of the theory through; the observation of endosymbiosis in nature, organelle function, modern advancements in genomics, DNA sequencing and the fossil record. The evidence against the theory will also be assessed. Both sides of the argument will be considered in establishing how well the evidence supports or refutes the theory that endosymbiosis is responsible for the origins of organelles in Eukaryotic cells.

 

Symbiosis and endosymbiosis in nature

Numerous life forms on this planet are committed to a cooperative relationship; from the bacteria in the dog’s stomach to Ophrydium, a ciliate that accommodates a colony of symbiotic bacteria who function as a single individual, (Margulis, 1998a). Termites depend on the micro-organisms that live in their guts to digest wood and in return the micro-organisms depend on the termites to find wood, (Dawkins, 1999). Recently two scientists, (Okamoto – Inouye, 2005) have observed the early stages of endosymbiosis in Hatena, a flagella found in Japan that has acquired a green “placid” and both, the host and the symbiont, have undergone changes to suit one another. Most importantly the placid is only inherited by one daughter cell, confirming that it is a symbiont, (Okamoto – Inouye, 2005). The fact that organism migrate into cooperation with other organisms is an observed phenomenon in nature; the question is whether the organelles of Eukaryotic cells had a symbiotic origin.

 

Early observations

In his book Lectures on the Physiology of Plants, Sachs (1882, cited by U. Kutschera) proposed that chloroplasts are independent organisms capable of reproducing independently of the cells nucleus. Almost a decade later, Altman (1890, cited by U. Kutschera et al) observed similarities between mitochondria and free living bacteria. Endosymbiosis , as an explanation for the origins of organelles in Eukaryotic cells, remained highly speculative in the scientific community until the Russian botanists, (C. Mereschkowsky, 1905a) published a landmark paper presenting three lines of evidence in favor of endosymbiosis: (1) chloroplast are transmitted from generation to generation and are never made new from the nucleus of the cell, (2) the close resemblance between plastids and free living cyanobacteria and (3) the resemblance of chloroplast and zoochlorellae (bacteria that live in other organisms). The advancement in genetics has further increased the evidence for the bacterial origins of chloroplast and mitochondria.

 

Bacterial ancestry and genetic evidence

Despite the hundreds of millions of years spent in the host cell, the chloroplast endosymbiot retains the characteristics of its bacterial ancestry, (McFadden, 1999). Mitochondria and chloroplast retain “genes, metabolic activities, genetic mechanisms and protein import complexes” that clearly establishes their prokaryotic origins, (Osteryoung and Nunnari, 2003). The DNA of the organelles indicates that they are more closely related to prokaryotic cells than their Eukaryotic hosts, (Archibald, 2005). Further analysis of DNA data has identified two groups of bacteria, α-Proteobacteria and Cyanobacteria as the closest relatives of mitochondria and chloroplast, respectively, (Gray-Spencer, 1996). This evidence has further strengthened the case for a bacterial ancestry of the organelles (Douglas and Raven, 2003). Plastids and mitochondria do not just resemble bacteria; they behave like bacteria as well. (Mereschkowsky, 1905) They grow, divide and produce substances synthetically without dependents on the nucleus of the cell, (Mereschkowsky, 1905b). However, the organelles are not completely independent of the nucleus. They have surrendered their metabolic functions to the host cell, (Margulis, 1970).

 

 

 

Cooperation with the host

DNA transfer from the nucleus to organelle is a, frequent, ongoing process which has been observed in laboratory experiments, (Huang, 2003). Genetic analysis has established that the eukaryotic cell nucleus contains ancient bacterial genes that have been transferred from the plastid organelles, (Archibald, 2005). How this protein chain evolved is one of the biggest mysteries of endosymbiosis. Margulis (1970) suggests that the same natural selection pressure that drives cave animals to lose their eyes would drive the endosymbiot to develop cooperation with the host nucleus, and vice-versa.

 

Fossils record

Another piece of evidence in favor of endosymbiosis is the fossil record. No fossil evidence has been found of intermediates between placid-lacking organisms and plastid containing organisms, as would be expected if the organelle evolved by slow accumulation of random mutation. However if the plastid merged with the host via endosymbiosis no such intermediated would exist (Margulis, 1970). This observation agrees with the theory of “Punctuated Equilibrium” which claims that evolution is not a gradual process but a process of stops and spurs, and that species can appear suddenly ( in evolutionary time) (Gould, 1980).

 

 

Critiques of the theory

The opposite view of endosymbiosis is the autogenous hypothesis, which claims that the organelles evolved inside the cell and were never free organisms (Cavalier-Smith. 1975, cited by U. Kutschera et al). It is also argued that not enough is known about the DNA makeup of mitochondria and plastids, and until further research is done the origins of the organelles in eukaryotic cells will remain a mystery. (Lang et al, 1999). Even though the fossil record seems to support the theory of endosymbiosis, the gradualist view points out how unlikely it is for organisms (especially bacteria) to fossilise and that the ‘gaps’ in the fossil record exist because animals failed to fossilise, not because it reflects what really happened in evolutionary history (Dawkins, 1996).

 

Conclusion

Through the observation of endosymbiosis in nature, DNA sequencing of the organelles which indicates a bacterial origins and the fossil record evidence in support of endosymbiosis, the puzzle of how mitochondria and chloroplast originated in Eukaryotic cells is starting to come together. Even though many aspects of the theory remain unknown, no alternative hypothesis can fit the evidence as well as endosymbiosis, (Martin-Kowallik, 1999). Darwin proposed that complexity is the result of billions of years of evolution. It was Darwin’s ‘bulldog’ Huxley who was skeptical of this view. Huxley believed evolution could progress so rapidly that the slow process of rock sedimentation would never catch the act. If endosymbiosis did occur in eukaryotic cells, it would have been an instant event and a truly giant leap in evolution of life on earth. The British geologist, Derek. V. Agar wrote: “The history of any one part of the earth, like life of a soldier, consists of long period of boredom and short periods of terror.” With the current evidence supporting endosymbiosis, he was probably right.

 

 

References

 

Altmann, 1890 R. Altmann, Die Elementarorganismen und ihre Beziehungen zu den Zellen, Verlag von Veit & Comp., Leipzig (1890). cited by U. Kutschera and K.J. Niklas , 2005 Endosymbiosis, cell evolution, and speciation vol. 124 pp. 1-24

 

 

Archibald, M.J, 2005, ‘Jumping Genes and shrinking Genomes – Probing the Evolution of Eukaryotic Photosynthesis with Genomics ‘, Life vol. 57, pp. 539-547

 

Dawkins R, 1999 ‘Unweaving the rainbow’ Penguin Books pp. 229-230

 

Dawkins R, 1996 ‘The Blind Watchmaker’ W.W Norton pp.223-225

 

Douglas A E., Raven J.A., 2003 ‘Genomes at the interface between bacteria and organelles’ Phil. Trans. R. Soc. Lond. B 358 (2003), pp. 5–18

 

Cavalier-Smith, 1975 T. Cavalier-Smith, The origin of nuclei and of eukaryotic cells, Nature 256 (1975), pp. 463–468. ) cited by U. Kutschera and K.J. Niklas , 2005 Endosymbiosis, cell evolution, and speciation vol. 124 pp. 1-24

 

Gray MW, Spencer DF. 1996. Organellar evolution. In Evolution of Microbial Life,ed. DM Roberts, P Sharp, G Alderson, MCollins, pp. 109–126. Cambridge: CambridgeUniv. Press

 

Gould S. J, 1980 ‘ The Episodic Nature of Evolutionary Change’ The Panda’s Thumb W.W Norton , pp. 179.186

 

Huang, C. Y., Ayliffe, M. A., and Timmis, (2003) ‘Direct measurement of the transfer rate of chloroplast DNA into the nucleus.’ Nature vol. 422 72-76.

 

Lang et al., 1999 B.F. Lang, M.W. Gray and G. Burger, Mitochondrial genome evolution and the origin of eukaryotes, Annu. Rev. Genet. 33, pp. 351–397.

 

Lynn Margulis (1998), The Symbiotic Planet: a new look at evolution, First edition, Published by Weidenfeld & Nicholson, London

 

Lynn Margulis (1970) ‘Origin of Eukaryotic Cells’ Yale University Press

 

 

Mereschkowsky, C. (1905). U $ ber Natur und Ursprung der

Chromatophoren im P¯anzenreiche. Biol. Centralbl., 25: 593±604. English

translation in Martin, W., Kowallik, K. V. (1999). Annotated English

translation of Mereschkowsky's 1905 paper `U $ ber Natur und Ursprung der

Chromatophoren im P¯anzenreiche '. Eur. J. Phycol., 34: 287±295.

 

 

 

Martin W, Kowallik K , (1999) ‘). Annotated English

translation of Mereschkowsky's 1905 paper `U $ ber Natur und Ursprung der

Chromatophoren im P¯anzenreiche '. Eur. J. Phycol., 34: 287±295.

 

Osteryoung and J. Nunnari 2003 K.W., The division of endosymbiotic organelles, Science 302, pp. 1698–1704.

 

Sachs, 1882 J. Sachs, Vorlesungen über Pflanzen-Physiologie, Verlag W. Engelmann, Leipzig (1882) cited by U. Kutschera and K.J. Niklas , 2005 Endosymbiosis, cell evolution, and speciation vol. 124 pp. 1-24

[CharonY]

I do not have the time atm too respond it much detail but firstly a stylistic question, is this supposed to be a "scientific" essay, kind like a mini-review or more in the line of a standard school essay?

Some quick points:

-introduction: the formation of eukaryotic cells by endosymbiosis is a special event, you tackle it too broadly in my view. Especially bringing in Darwin appears awkward

-in your second paragraph you skim briefly over some examples of symbiosis and then throw endosymbiosis into it- rather give a definition of symbiosis and point out endosymbiosis as a special form of it. There are other forms of endosymbiosis.

 

You talk about endosymbiosis in general but mostly only mention plastids, then you switch at some points and talk about endosymbiosis in general. For such a short essay better focus just on one point (e.g. plastids) and explicitly state that.

You forgot to mention the one of the most important points in favour of the theory of endosymbiosis is the bacterial cell hull components of plastids and mitochondria.