Arthropod endosymbiosis and evolution

The association of two species that live on or in one another was first described in the nineteenth century, and the word symbiosis was proposed to denote this biological phenomenon (Sapp 1994). The discovery that lichens are organisms generated by the integration of a fungus and blue-green algae, that is, cyanobacteria, was followed by a number of other studies that have shown how the association of different species is widespread in nature and characterized by different degrees of benefit-sharing. Symbiosis encompasses both antagonistic relationships, in which one organism takes advantage of the other, and mutualistic relationships, where both partners gain advantage from their association. There are also cases where no clear benefit or harm is evident for both interacting species, which are then, in some cases, considered commensals. The term symbiosis applies to all these type of species associations, and not only to mutualism, as is sometimes erroneously done (Sapp 1994).