Endosymbiosis and The Origin of Eukaryotes

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The endosymbiosis theory postulates that

The Evidence

The Mitochondrial Genome

The genome of human mitochondria contains 16,569 base pairs of DNA organized in a closed circle. These encode: The 13 polypeptides participate in building several protein complexes embedded in the inner mitochondrial membrane.

All these gene products are used within the mitochondrion, but the mitochondrion also needs >900 different proteins as well as some mRNAs and tRNAs encoded by nuclear genes. The proteins (e.g., cytochrome c and the DNA polymerases used within the mitochondrion) are synthesized in the cytosol and then imported into the mitochondrion.

The Chloroplast Genome

The genome of the chloroplasts found in Marchantia polymorpha (a liverwort, one of the Bryophyta) contains 121,024 base pairs in a closed circle. These make up some 128 genes which include: All these gene products are used within the chloroplast, but all the chloroplast structures also depend on proteins RUBISCO, for example, the enzyme that adds CO2 to ribulose bisphosphate to start the Calvin cycle, consists of multiple copies of two subunits:

The arrangement of genes shown in the figure is found not only in the Bryophytes (mosses and liverworts) but also in the lycopsids (e.g., Lycopodium and Selaginella). In all other plants, however, the portion of DNA bracketed by the red arrows on the left is inverted. The same genes are present but in inverted order. The figure is based on the work of Ohyama, K., et al., Nature, 322:572, 7 Aug 1986; and Linda A. Raubeson and R. K. Jansen, Science, 255:1697, 27 March 1992.

The evolution of the eukaryotic chloroplast by the endosymbiosis of a cyanobacterium in a mitochondria-containing eukaryotic host cell led to the evolution of

Secondary Endosymbiosis: Eukaryotes Engulfing Eukaryotes

The Nucleomorph

Once both heterotrophic and photosynthetic eukaryotes had evolved, the former repeatedly engulfed the latter to exploit their autotrophic way of life. Many animals living today engulf algae for this purpose [Link to examples]. Usually the partners in these mutualistic relationships can be grown separately.

However, a growing body of evidence indicates that the chloroplasts of some algae have not been derived by engulfing cyanobacteria in a primary endosymbiosis like those discussed above, but by engulfing photosynthetic eukaryotes. This is called secondary endosymbiosis. It occurred so long ago that these endosymbionts cannot be cultured away from their host.

In two groups, the eukaryotic nature of the endosymbiont can be seen by its retention of a vestige of a nucleus (called its nucleomorph).

The result in both cases: a motile, autotrophic cell containing:

The Four Genomes of Guillardia theta

The cryptomonad Guillardia theta contains four different genomes:

Susan Douglas and her colleagues reported (in the 26 April 2001 issue of Nature) the completely-sequenced genome of the nucleomorph.

It contains 3 small chromosomes with The genes are crowded closely on the three chromosomes. In fact, 44 of them overlap each other. Only 17 genes contain introns, and these are very small.

Genome Interactions in Guillardia theta

Millions of years of evolution have resulted in a complex but precisely-orchestrated array of interactions between the 4 genomes. For example:

The Apicoplast

The apicoplast (short for "apicomplexan plastid") is a solitary organelle found in the apicomplexan protists: "sporozoans" like Plasmodium falciparum (and the other agents of malaria) and Toxoplasma gondii.


The apicoplast is the product of an ancient endosymbiosis in which the eukaryotic ancestor engulfed a unicellular alga — probably a red alga — with a solitary chloroplast. Over time, the nucleus was lost (no residual nucleomorph) as well as many features of the chloroplast (including its ability to perform photosynthesis).

Can Secondary Symbiosis Still Occur?

Probably so.

Two Japanese scientists have discovered a heterotrophic flagellate that engulfs a unicellular green alga that lives freely in the surrounding water. Once inside,

When the host divides by mitosis, only one daughter cell gets the plastid. The other cell regrows the feeding apparatus and is ready to engulf another alga.

You can read the details in Okamoto, N. & Inouye, I., Science, 310:287, 14 October 2005.

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25 February 2016