Endosymbiosis: Lessons in Conflict Resolution

PLoS Biology | http://biology.plosjournals.org Symbiosis, an interdependent relationship between two species, is an important driver of evolutionary novelty and ecological diversity. Microbial symbionts in particular have been major evolutionary catalysts throughout the 4 billion years of life on earth and have largely shaped the evolution of complex organisms. Endosymbiosis is a specifi c type of symbiosis in which one—typically microbial—partner lives within its host and represents the most intimate contact between interacting organisms. Mitochondria and chloroplasts, for example, result from endosymbiotic events of lasting signifi cance that extended the range of acceptable habitats for life. The wide distribution of intracellular bacteria across diverse hosts and marine and terrestrial habitats testifi es to the continued importance of endosymbiosis in evolution. Among multicellular organisms, insects as a group form exceptionally diverse associations with microbial associates, including bacteria that live exclusively within host cells and undergo maternal transmission to offspring. These microbes have piqued the interest of evolutionary biologists because they represent a wide spectrum of evolutionary strategies, ranging from obligate mutualism to reproductive parasitism (Buchner 1965; Ishikawa 2003) (Box 1; Table 1). In this issue of PLoS Biology, the publication of the full genome sequence of the reproductive parasite Wolbachia allows the fi rst genomic comparisons across this range (Wu et al. 2004). Lifestyle Extremes in Insect Endosymbionts