21.1: Making eukaryotes from prokaryotes

Figure \(\PageIndex{1}\): An interpreted “tree of life” showing proposed relationships between the two domains of prokaryotes (Bacteria and Archaea) and the Eukaryotic line. Note the two highlighted episodes of endosymbiosis.

Eukaryotic cells are the large, complicated cells that make up the bodies of animals, plants, fungi, and protists. With their well-defined nucleus, they are distinct from the smaller, simpler cells that comprise the bodies of single-celled microbes. Microbial cells are prokaryotes instead, and come in two varieties: bacteria and archaea. Bacteria and Archaea strongly resemble one another in size and form (including their lack of a nucleus), but are distinguished from one another on the basis of utterly distinct genetics and biochemistry.

Though the domain Eukarya contains a diversity of forms, the defining characteristic of a eukaryote has long been recognized as its membrane-bound nucleus, the site where its genetic material is stored (in the form of DNA) and transcribed (in the form of RNA). The –karyotic part of the names “eukaryote” and “prokaryote” translates as kernel or nugget, a reference to the distinct visibility of the nucleus in those cells that have one. However, the mitochondrion could be considered just as defining and ubiquitous a structure for eukaryotes. Only a few anaerobic eukaryotes lack mitochondria. The structure, function, and genetic signature of the mitochondrion gives every indication that it represents the “capture” of a bacterial cell by an archeal cell sometime around 1.45 Ga (Mesoproterozoic).

Figure \(\PageIndex{2}\): A conceptual “tree of life” showing the relationship between Bacteria (purple), Eukarya (red), and several groups of Archaea (blues and greens). The archeal clade called “Asgard” is most closely related to line that would become eukaryotic host cells. Not shown is the endosymbiotic merger between bacteria and ur-Asgardians that allowed the eukaryotic line to innovate and flourish. (Modified from Figure 1 of Eme, et al. (2017).)

There is great diversity among the Archaea, and several major groups within that domain had split off already by the time one sub-group had some members engage in endosymbiosis while others didn’t. The group of Archaea most closely related to the eukaryotes are the superphylum Asgard. The Asgardians are more related to the “Eukaryotic” host cell line than they are to other groups classified as fellow archeons. Somewhere at or just after the branching point between Asgard and Eukarya was the moment when the first proto-mitochondrion was engulfed. The endosymbiosis between the Asgard-like archeon and its future mitochondrion became cemented into a permanent and irrevocable interdependency through geologic time. What was initially perhaps happenstance and casual eventually evolved into something permanent and obligatory, an innovation that was wildly successful in allowing diversification of life on Earth.

Endosymbiosis is often thought of as the larger organism “engulfing” the smaller, but it could just as legitimately be viewed as the “colonization” of the larger organism by the smaller one. Either way, each organism benefits from the relationship. The net result is a genetic chimera, a blending and a merger between two organisms that used to be distinct, but when combined make something new and different. All eukaryotes are chimeras, some several times over.

Figure \(\PageIndex{3}\): Components of a typical animal cell:
1) Nucleolus; 2) Nucleus; 3) Ribosome (dots as part of 5); 4) Vesicle; 5) Rough endoplasmic reticulum; 6) Golgi apparatus; 7) Cytoskeleton; 8) Smooth endoplasmic reticulum; 9) Mitochondrion; 10) Vacuole; 11) Cytosol (fluid that contains organelles; with which, comprises cytoplasm); 12) Lysosome; 13) Centrosome; 14) Cell membrane. (Public domain image via Wikimedia.)

What is true for mitochondria among all eukaryotes is also true for another organelle in some eukaryotes: plants and the various forms of “algae” (which are protists) contain chloroplasts, organelles that have the ability to conduct photosynthesis. In structure, function, and genes, the chloroplasts resemble free-living cyanobacteria, and are thus thought to represent a second episode of endosymbiosis unique to the photosynthetic eukaryotes.

In both cases, the smaller symbiont retains its own DNA, and reproduces independently of the rest of the host cell. Over geologic time, the genome of the internal symbiont will be pared down to eliminate redundancy, with the genome sometimes getting trimmed down to a third the size of the length of the DNA in their free-living relatives. Some of its genes may be copied or moved into the host genome. The two organisms may become locked into interdependency, where neither can survive without the other. At this point, it starts to make more sense to refer to the internal symbiont not as an “organism” in its own right, but as an organelle within the host cell.

There are two major organelles in eukaryotic organisms that have resulted from endosymbiosis, mitochondria (common to most eukaryotes) and chloroplasts (found in plants and algae only), as well as a few less obvious examples. After reviewing the scientific history of the idea, this case study will examine each organelle in turn.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Which organelles in eukaryotes are thought to have started due to endosymbiosis of bacteria in archaea?

a. Mitochondria and vacuoles

b. Mitochondria and chloroplasts

c. Vacuoles and chloroplasts

d. The Golgi apparatus and mitochondria

e. Vacuoles and the Golgi apparatus

f. Chloroplasts and the Golgi apparatus

Answer

b. Mitochondria and chloroplasts