Symbiosis – What is it?
The term symbiosis relates to close interactions between different biological organisms and the interdependent relationships between living things, in which completely different forms of life depend upon each other for existence. One example of beneficial symbiosis (or mutualism) is found between algae and the fungus of lichens. While fungi provide vital protection and moisture to algae, the algae nourish the fungi with photosynthetic nutrients that keep them alive.
Another example of mutual symbiosis is the relationship which exists between the ocellaris clownfish that dwells among the tentacles of Ritteri sea anemones. The territorial fish protects the anemone from carnivorous fish, and in turn the stinging tentacles of the anemone protect the clownfish from its predators. A special mucus on the clownfish protects it from the stinging tentacles.
Plants that are pollinated by insects have highly specialized flowers modified to promote pollination by a specific pollinator that is also correspondingly adapted. Many herbivores have mutualistic gut fauna that aid them in the digestion of plant matter. Coral reefs are the result of mutualistic symbiotic relationships existing between coral organisms and various types of algae that live inside them. Most land plants and land ecosystems rely on mutualisms between the plants which fix carbon from the air, and fungi which help in extracting minerals from the ground.
Symbiosis – A challenge to evolution?
Darwin’s theory of biological change was based upon competition among the individuals making up a species. In The Origin, Darwin concedes that “If it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not have been produced through natural selection.”
How can plants that require certain animals to survive have existed before those animals appeared in the first place? Moreover, how do animals that need other animals to survive arrive without their partners arriving at the exact same moment?
Symbiosis and endosymbiosis
A popular theory often cited in reference to the origin of intracellular organelles is the principle of endosymbiosis. This hypothesis proposes that chloroplasts and mitochondria began as freeliving aerobic prokaryotic ancestors which were engulfed by an ancient prokaryotic cell. These endosymbionts eventually became the intracellular organelles, which then apparently lost many of their own genes to the nuclei of their hosts. However, how such a stable relationship between ingested aerobic invaders and an anaerobic host was possible -- and why some specific genes and not others should be transferred to the host’s nucleus -- has yet to be documented. It seems highly unlikely that the cell conveniently developed the transport pathways to return the organelle proteins back to the relevant organelle. With no mechanism by which the pathway could be formed, the organelles would become obsolete once the transfer of genes to the nucleus was initiated. Furthermore, the organelles would likely cease to function because they would not confer a selection advantage.
An insight into how many genes were lost to the host nucleus may be derived from the fact that the cytosol synthesises for the mitochondria the following proteins: aminoacyl-tRNA synthases; DNA replication enzymes; RNA polymerase; soluble enzymes of the citric acid cycle etc. It is clear that, since proteins are made at two independent sites, nuclear-coded proteins must be imported into mitochondria and chloroplasts. The principle difficulty lies in the fact that imported proteins have to cross subcompartments to get into both organelles as the organelles possess double membranes. Here is where chaperones are required to bind the polypeptide chains just as they emerge through spacial pores into the mitochondrial matrix. A similar process operates in the importing of proteins into the chloroplast. Because platn cells possess both chloroplasts and mitochondria, two different kinds of signal peptides are also required to send proteins to the correct addresses.
The immensely sophisticated transport arrangements raises the question as to how they arose and what selective advantages there would have been in relation to the original endosymbionts to share genomes with the nucleus of the host cell. As if this is not difficult enough, a further logistical problem is created by the fact that all of the host cell’s fatty acids and a number of amino acids are made by enzymes in the chloroplast stroma. We have now a transfer in reverse. Of course endosymbiosis could only take place when cells with highly developed metabolic systems were in existence.
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