Symbiotic relationships can be view as beneficial to both organisms or harmful to one of the organisms. Parasitism involves one organism living completely at the expense of the other, whereas mutualism involves mutual benefit to both partners. Parasitism has a gradation from pathogens that seriously incapacitate or kill their hosts to microorganisms that consume so little of the host's resources that their presence is scarcely significant. Mutualism also shows a gradation, from relationships in which both organisms can survive without each other to relationships that are completely co-dependent.
Many different examples of mutualistic and parasitic symbiotic relationships exist. Mutualistic associations were perhaps responsible for the origin of mitochondria and chloroplasts from free-living prokaryotes. Rhizobium invades the plant roots of the pea family where it builds a nodule or small bump to inhabit. Rhizobium receives nutrients from the plant to survive, and in exchange, it converts nitrogen from the soil into a form the plant can readily use. Consequently, this is an example of a mutualistic symbiosis in which both partners benefit. We have already discussed Agrobacterium (Ch. 16), which infects plant cells and transfers T-DNA into the plant cell nucleus. Much like Rhizobium, the Agrobacterium induces the plant to make a mass of cells for the bacteria to inhabit. Unlike Rhizobium, the Agrobacterium does not provide any service for the plant, and simply uses the plant to provide food. This is a simple example of parasitism.
We shall consider two more examples where bacteria live inside eukaryotic cells to the benefit of both partners. In the case of Paramecium and Caedibacter, the bacteria are only slightly modified and some can still live independently. In the case of Buch-nera and the insects they inhabit, the bacteria are so modified that they may almost be regarded as a specialized biosynthetic organelle.
Killer Paramecium contains symbiotic bacteria that make the toxin for killing sensitive strains of Paramecium.
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