Endosymbiosis - The Appearance of the Eukaryotes


The Endosymbiotic Origin of Domain Eukaryota

The Eukaryotes

Also see:
Domain Eukaryota
Domains Archaea and Bacteria
Evolutionary Origin of Eukaryotic Cells
Archaea and Evolution

Endosymbiotic Theory Introduction

Electron Microscope Image of MitochondionThe hypothesized process by which prokaryotes gave rise to the first eukaryotic cells is known as endosymbiosis, and certainly ranks among the most important evolutionary events. Endosymbiotic theory, that attempts to explain the origins of eukaryotic cell organelles such as mitochondria in animals and fungi and chloroplasts in plants was greatly advanced by the seminal work of biologist Lynn Margulis in the 1960s. Mitochondria are one of the many different types of organelles in the cells of all eukaryotes. In general, they are considered to have originated from proteobacteria (likely Rickettsiales) through endosymbiosis. Chloroplasts are one of the many different types of organelles in the plant cell. In general, they are considered to have originated from cyanobacteria through endosymbiosis. Endosymbiosis has gained ever more acceptance in the last half century, especially with the relatively recent advent of sequencing technologies. There are many variants to the theory, regarding what organism(s) engulfed what other organism(s), as well as how many times and when it occurred across geological time. The biology is messy, and there are many competing theories, so here we tend to converge them in a unified conceptualization [for more detailed treatment, visit the "Origins of the Eukara" pages at Palaeos].

Symbiosis and Co-evolution

Eukaryote and Prokaryote Cell ComparisonSymbiosis is ubiquitous among organisms throughout the tree of life, from the species level to the kingdom level, and even to the domain level. It is integral to evolution as cooperating organisms gain survival advantage by a quid pro quo between them. For example, you (and for that matter all herbivores omnivores) could not digest your food without the exquisite symbiosis between your gut and the bacteria therein. Symbiosis played a major role in the co-evolution of flowering plants and the animals that pollinate them. The fossil record indicates that the first flowering plants had primitive flowers. Through natural selection, adaptive speciation quickly gave rise to many diverse groups of plants with specialized, and, at the same time, corresponding speciation occurred in certain insect groups. Many plants are pollinated by insects and vertebrates (e.g., bats and or birds) that have evolved highly specialized flowers facilitating pollination by a specific group or species that are themselves concomitantly adapted through co-evolution. Such mutualistic associations, where both host and symbiont evolve to accommodate one another abound in the history of life. But, we digress, so let's return to endosymbiosis. The flower-pollinator relationship is a common example of symbiosis and resultant co-evolution. Many flowers have close relationships with one or a few specific pollinating organisms. Many flowers, for example, attract only one specific species of insect, and therefore rely on that insect for successful reproduction. This close relationship is often given as an example of coevolution, as the flower and pollinator are thought to have developed together over a long period of time to match each other's needs. This close relationship compounds the negative effects of extinction. The extinction of either member in such a relationship would mean almost certain extinction of the other member as well. Some endangered plant species are so because of shrinking pollinator populations.

Endosymbiotic Theory and Eukaryotic Origins

Such symbiotic relationships in which two species are dependent upon one another to varying extents also served as crucial elements of the evolution of eukaryotic cells. The theory holds that the eukaryote mitochodrion evolved from a small, autotrophic bacterium that was engulfed by a larger primitive, heterotrophic, eukaryotic cell. This eukaryotic cell arose when an
anaerobic prokaryote (unable to
use oxygen for energy) lost its cell wall. The more flexible membrane underneath then began to grow and fold in on itself which, in turn, led to
formation of a nucleus and other internal membranes. Endosymbiosis occurred according to the figure to the right: a) The primitive eukaryotic cell was also eventually able to eat prokaryotes, a marked improvement to absorbing small
molecules from its environment; b) The process of endosymbiosis commenced when the eukaryote engulphed but did not digest a autotrophic bacterium. Evidence suggests this engulfed bacterium was an alphaproteobacteria, an autotroph that uses photosynthesis to acquire energy. c) The eukaryote then began a mutually beneficial (symbiotic) relationship with it whereby the eukaryote provided protection and nutrients to the prokaryote, and in return, the prokaryotic endosymbiont provided additional energy to its eukaryotic host through its respiratory cellular machinary. d) The relationship became permanent over time completing primary endosymbiosis as the endosymbiont lost some genes it used for independent life and transferred others to the eukaryote's nucleus. The symbiont thus became dependent on the host cell for organic molecules and inorganic compounds. The genes of the repiratory machinary became a mitochondrion. Endosymbiotic theory hypothesizes the origin of chloroplasts similarly, where chloroplasts a eukaryote with mitochondria engulfs a photosynthetic cyanobacteruim in a symbiotic relationship ending in the chloroplast organelle.

When these endosymbiotic events occured is subject to much debate, but it must have been early in life's history, perhaps as early as the Archean Eon more than 2500 million years ago. The heterotrophic prokaryote used cellular respiration to intake oxygen and convert organic molecules to energy. The prokaryotic cells that were too small to be digested continued to live inside the host eukaryote, eventually becoming dependent on the host cell for organic molecules and inorganic compounds. Importantly, the host cell could have acquired, by the addition of the aerobic function, an increased output of ATP for cellular activities, leading to its improved selective advantage. Was the "engulfer" a eubacteria or an archaean - yes - it depends on which of competing theories you choose? Other theories hold that the prokaryotes that gave rise to early eukaryotes were probably from the Domain Archaea, both because of several key characteristics and because DNA sequence comparison suggest that archaeans are more closely related to the eukaryotes than are eubacteria. This is the so-called serial endosymbiosis theory of a monophyletic origin of the mitochondrion from a eubacterial ancestor. That fact that mitochondria have their own DNA, RNA, and ribosomes, supports the endosymbiosis theory, as does the existence of the amoeba, a eukaryotic organism that lacks mitochondria and therefore requires a symbiotic relationship with an aerobic bacterium.

Endosymbiosis Leads to Mitochondria

Eukaryotic Animal CellDigging deeper, the symbiosis is analogous to that between plants and their "birds and bees" symbionts. The aerobic bacterium thrived within the cell cytoplasm that provided abundant molecular food for its heterotrophic existence. The bacterium digested these molecules that manufactured enormous energy in the form of adenosine triphosphate (ATP), and so much so that extra ATP was available to the host cell's cytoplasm. This enormously benefited the anaerobic cell that then gained the ability to aerobically digest food. Eventually, the aerobic bacterium could no longer live independently from the cell, evolving into the mitochondrion organelle. Such aerobically obtained energy vastly exceeded that of anaerobic respiration, setting the stage for vastly accelerated evolution of eukaryotes.


Endosymbiosis Leads to Chloroplasts

Eukaryotic Plant CellEndosymbiotic theory posits a later parallel origin of the chloroplasts; a cell ate a photosynthetic cyanobacterium and failed to digest it. The cyanobacterium thrived in the cell and eventually evolved into the first chloroplast. Other eukaryotic organelles may have also evolved through endosymbiosis; it has been proposed that cilia, flagella, centrioles, and microtubules may have originated from a symbiosis between a Spirochaete bacterium and an early eukaryotic cell, but this is not yet broadly accepted among biologists.



Secondary Endosymbiosis

Primary endosymbiosis involves the engulfment of a bacterium by another free living organism. Secondary endosymbiosis occurs when the product of primary endosymbiosis is itself engulfed and retained by another free living eukaryote. Secondary endosymbiosis has occurred several times and has given rise to extremely diverse groups of algae and other eukaryotes. Some organisms can take opportunistic advantage of a similar process, where they engulf an alga and use the products of its photosynthesis, but once the prey item dies (or is lost) the host returns to a free living state. Obligate secondary endosymbionts become dependent on their organelles and are unable to survive in their absence. The process of secondary endosymbiosis left its evolutionary signature within the unique topography of plastid membranes. Secondary plastids are surrounded by three (in euglenophytes and some dinoflagellates) or four membranes (in haptophytes, heterokonts, cryptophytes, and chlorarachniophytes). The two additional membranes are thought to correspond to the plasma membrane of the engulfed alga and the phagosomal membrane of the host cell. The endosymbiotic acquisition of a eukaryote cell is represented in the cryptophytes; where the remnant nucleus of the red algal symbiont (the nucleomorph) is present between the two inner and two outer plastid membranes. Despite the diversity of organisms containing plastids, the morphology, biochemistry, genomic organisation, and molecular phylogeny of plastid RNAs and proteins suggest a single origin of all extant plastids – although this theory is still debated. Some species including Pediculus humanus have multiple chromosomes in the mitochondrion. This and the pylogenetics of the genes encoded within the mitochondrion suggests that the ancestors of mitochondria may have been acquired on several occasions rather than just once.

Mitochondria and Chloroplasts Cell Powerhouses

We could fairly posit that the evolutionary origin of the eukaryotic cell was "the first time that what went around came around", a quid pro quo with among primitive organisms in deep time. This would make the all eukaryotes chimaeras at a cellular level. The Eukaryotic cell could also be likened to the V8 engine in producing power, as compared to a donkey powering prokaryotic cells. This would have enormous implication for subsequent evolution as earth's oceans atmosphere were oxygenated by photosynthetic bacteria creating extensive stromatolitic reefs. Eukaryotes became multicellular in the precambrian at the same time earth's oxygen levels were rising. More oxygen together with mitochondria to burn it in was one driving function for the forthcoming Cambrian Explosion when the ancestors of modern eukaryotes' appeared. Mitochondria, the result of endosymbiosis in eukaryotic evolution are the energy-generating V8 engines of eukaryotic cells, where oxidative phosphorylation and electron transport metabolism takes place. Plastids, including chloroplasts, are the corresponding photosynthetic organelles of plant and algae cells.

Mitochondrial DNA and Function

Mitochondrial DNA and FunctionThe mitochondrion is different from most other organelles because it has its own circular DNA (similar to the DNA of prokaryotes) and reproduces independently of the cell in which it is found, one of the major pieces of evidence supporting endosymbiosis. Although most DNA is packaged in chromosomes within the nucleus, mitochondria also have a small amount of their own DNA. This genetic material is known as mitochondrial DNA or mtDNA. Mitochondria are structures within cells that convert the energy from food into a form that cells can use. Each cell contains hundreds to thousands of mitochondria, which are located in the fluid that surrounds the nucleus (the cytoplasm). Mitochondria produce energy through a process called oxidative phosphorylation. This process uses oxygen and simple sugars to create adenosine triphosphate (ATP), the cell’s main energy source. A set of enzyme complexes, designated as complexes I-V, carry out oxidative phosphorylation within mitochondria. In addition to energy production, mitochondria play a role in several other cellular activities. For example, mitochondria help regulate the self-destruction of cells (apoptosis). They are also necessary for the production of substances such as cholesterol and heme (a component of hemoglobin, the molecule that carries oxygen in the blood). Human mitochondrial DNA contains 37 genes, all of which are essential for normal mitochondrial function. Thirteen of these genes provide instructions for making enzymes involved in oxidative phosphorylation. The remaining genes provide instructions for making molecules called transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs), which are chemical cousins of DNA. These types of RNA help assemble protein building blocks (amino acids) into functioning proteins.

Chloroplast DNA and Function

Like mitochondria, chloroplasts also contain their own DNA and are able to grow and reproduce independently within the cells of plants and other protista that conduct the complex chemistry in photosynthesis. In green plants, chloroplasts are surrounded by two lipid-bilayer membranes thought to correspond to the outer and inner membranes of the ancestral cyanobacterium from which chloroplasts descended. Chloroplasts have their own genome, which is considerably reduced compared to that of free-living cyanobacteria, but the parts that are still present show clear similarities with the cyanobacterial genome. Plastids typically contain some 60 to 100 genes, compared cyanobacteria that have some 1500 genes. Many of the apparently missing genes are encoded in the nuclear genome of the host. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.

Evidence for Endosymbiotic Theory

  • Mitochondria have very similar characteristics to purple-aerobic bacteria. They both use oxygen in the production of ATP, and they both do this by using the Kreb’s Cycle and oxidative phosphorylation. Similarly, chloroplasts are very similar to photosynthetic bacteria in that they both have similar chlorophyll that harnesses light energy that is converted into chemical energy. Although there are many similarities between mitochondria and purple aerobic bacteria and chloroplasts and photosynthetic bacteria, they appear to be slight and explainable by subsequent evolution.
  • Mitochondria and chloroplasts are similar in size to bacteria, 1 to 10 microns.
  • Mitochondria and chloroplasts DNA, RNA, ribosomes, chlorophyll (for chloroplasts), and protein synthesis is similar to that for bacteria. This provided the first substantive evidence for the endosymbiotic hypothesis. It was also determined that mitochondria and chloroplasts divide independently of the cell they live in. Mitochondria having their own DNA and dividing independently of the cell is what ultimately results in only mitochondrial DNA being inherited by one’s mother since only an egg cell has DNA while a sperm cell does not.
  • Both mitochondria and chloroplasts have double phospholipid bilayers. This appears to have arisen by mitochondria and chloroplasts entering eukaryotic cells via endocytosis. Both purple, aerobic bacteria (similar to mitochondria) and photosynthetic bacteria (similar to chloroplasts) only have one phospholipid bilayer, but when they enter another cell via endocytosis, they are bound by a vesicle which forms the second layer of their double phospholipid bilayer.