ABSTRACT
Introduction Relationships where two organisms engage in close metabolic or cellular interactions can potentially lead to mutually benecial coexistence (Baumann, 2005). However, symbiotic coexistence is never the primary aim of these interactions, but the result of an evolutionary race, where both organisms continuously adapt to stay in the relationship (Red Queen effect). To be of mutual benet for both organisms, both sides have to give up established pathways to engage in novel modes of action that give the new entity an overall selective advantage (Kitano and Oda, 2006). Although there is no room for compromise between the inherent virulence of the intruding organism and the defense abilities of the host insect, it is unlikely that symbiotic relationships are established if one side always wins. Only if both interacting organisms change and evolve in the process is the emergence of a truly symbiotic relationship possible. From an immunological point of view, these relationships are an evolutionary trade-off between the selective advantages gained by the two coexisting organisms versus the loss of individual integrity and independence of the host insect and the intruder. This raises the question of how we dene host integrity and why insects are ghting some intruders but not others. In fact, to describe how two organisms evolve to coexist in symbiotic relationships, we have to understand how multicellular organisms such as insects recognize “self” from “nonself.” This also raises the interesting question of whether multicellular organisms are in fact the result of symbiotic relationships of genetically identical but potentially independent cells.
