The Endosymbiont Hypothesis

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An interesting question can produce some interesting answers. One of these is the question of what living things are made of. There are many possible answers, depending on who you ask. A young child asking his parents may simply be told: “Stuff”. A chemist may go into the detail of the basic common organic elements: Carbon, Hydrogen, Oxygen, Nitrogen and Phosphorous (Starr et al., 2009) and the different compounds that are formed. As biologists, we tend to look at the composition of living things at a relatively larger scale viz. from a scale of less than 0.1 nm to a scale of more than 1 nm (Starr et al., 2009a). I say relatively because, although this is still very small, it is much larger than the scale at which chemists work. This scale encompasses the realm of cells and cellular structures. The basic unit of life, as defined by the cell theory, pioneered by Schwann and Schleiden in 1838 (Mazzarello, 1999; Wayne, 2009), is the cell. Since that time, a lot of development has occurred in the quest to find the basis of life. It is now known that there are in fact two distinct kinds of cells, i.e. the prokaryotic and eukaryotic cells. The distinct differences between the two are easily discernible with modern technology such as electron microscopes. These differences include: the presence of a nucleus in the eukaryotes, and the absence of one in the prokaryotes; Eukaryotes also have distinct membrane-bound organelles, while prokaryotes do not (Stanier et al., 1970a; Mauseth, 2009). It is after discovery and examination of these organelles that first led to the Endosymbiont Hypothesis or Endosymbiont Theory, as it is sometimes called (both terms are used throughout). In this essay we shall look at the history of the theory, how it was formed, who the most influential collaborators were, and how it is applied to the evolution of certain organelles, in particular, the chloroplast and mitochondria. The development of the Endosymbiont Theory occurred over a period of more than 120 years, beginning in 1883, when Andreas Schimper, a biologist, noted that the way in which chloroplasts (he in fact coined the term) divide is very similar to the process of division in cyanobacteria. He proposed that these chloroplasts arose from the symbiosis of two separate organisms; one being the bacterium (Sapp et al., 2002; Torday et al., 2012), and the other, a host cell. This was backed up 30 years later by Constantin Mereschowsky when he confirmed that chloroplasts have similar characteristics to cyanobacteria (Wayne, 2009) (See Table 1). After the discovery and subsequent study of mitochondria, it was proposed by Ivan Wallin in 1922 (Wallin, 1925) that they too were formed by the symbiosis of bacteria. By this stage, the Endosymbiotic Theory was gaining popularity and began to be published in the more popular biology textbooks of the day (Wayne, 2009). Before we continue with the more modern additions to the theory let us examine what the term “endosymbiosis” actually entails. The word “endosymbiosis” can be broken into two separate entities, “endo-“and “-symbiosis”. “Endo-” is a prefix used when referring to something being internal (Hartmann-Petersen et al., 2007). Symbiosis, coined by Anton DeBary in 1879 (Wallin, 1923), is loosely defined as “a long-term relationship between two different species” (Encyclopedia Britannica 2009, 2009). This “long-term relationship” can be beneficial or harming to any one or both (if beneficial) symbionts, viz. the organisms associated in the relationship (Hartmann-Petersen et al., 2007). Thus, it becomes clear that the term refers to a symbiotic relationship between two organisms, where one lives inside the other. This internal organism is the “Endosymbiont”. Figure 1 below shows graphically how this happens.

Figure : Simplified diagram of endosymbiosis (Zhong et al., 2012) Since the discoveries of the late 1800’s and early 1900’s, very little progress was made towards the theory, and thus...
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