21.5: Other organelles

Though mitochondria and chloroplasts are the most well known and important examples of microbial endosymbiosis leading to novel organelles, there are perhaps other examples too.

Consider the flagellum, a whip-like appendage that projects off the surface of many groups of microbes. Lynn Margulis suggested that the flagellum appears to be an organelle that represents a merger between a spirochete bacterium and a host archeal cell related to the genus Thermoplasma. This idea hasn’t been as thoroughly validated as the endosymbiotic origins of mitochondria and chloroplasts, but it remains an intriguing idea worth further investigation.

Figure \(\PageIndex{1}\): Mixotricha paradoxa is “one” organism with five genomes. (Modified from an SEM image by Dean Soulia.)

Mixotricha paradoxa is an interesting example that appears to corroborate the idea of a symbiotic origin for flagella. Mixotricha is a species of protozoan (a eukaryote) that lives inside the gut of certain Australian termites. Unusually for protozoans, it sports both cilia and flagella, which is why its name translates to “a paradoxical mixture of hairs.” Mixotricha has five genomes: one for the main host cell, one for an endosymbiont that helps with the digestion of cellulose and lignin, and three on the surface (two spirochetes and one rod bacterium).

Some scientists have suggested that the nucleus itself, that iconic hallmark of the eukaryotic cell, arose through an even earlier episode of endosymbiosis. Perhaps that earliest, most fundamental merger occurred with a now-extinct group of microbes. Hartman and Federov (2002) identified hundreds of genes coding for proteins that are found in eukaryotic cells but have no equivalents in either Archaea or Bacteria. They interpret this as evidence of a third microbial line, the primordial host cell that would eventually lead to the eukaryotic line, dubbing it the Chronocyte (translation = time cell). Another possibility is that a large DNA virus may have been responsible; this is the viral eukaryogenesis hypothesis. Future research may shed more light on these intriguing possibilities.

An intriguing series of mergers and acquisitions permeate the history of eukaryotes on Earth, some more plain and others more obscure. When we consider ourselves as individuals, its humbling to realize we are in fact communities within communities, a whole that is greater than the sum of its parts.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Mixotricha is a protist with what surprising aspect to its anatomy?

a. It helps termites digest the cellulose in wood.

b. It is really large.

c. It is shockingly handsome for a eukaryote.

d. It is a symbiotic relationship between five distinct organisms, each with their own genome.

Answer

d. It is a symbiotic relationship between five distinct organisms, each with their own genome.

Exercise \(\PageIndex{2}\)

A traditional view of evolution is an endlessly-branching tree, with individuals competing fiercely against one another for limited resources. How does an endosymbiotic origin for key organelles bear on that idea?

a. It suggests that the only way to succeed is to outcompete every other organism.

b. It suggests another source of evolutionary novelty and success may be found through cooperation and community-building between formerly independent organisms.

c. It suggests that the traditional view is almost certainly (and exclusively) correct.

Answer

b. It suggests another source of evolutionary novelty and success may be found through cooperation and community-building between formerly independent organisms.