Role of Symbiosis in Evolution

Abstract

The biological relevance of the widespread prokaryote–eukaryote symbioses as a source of evolutionary innovation has been unveiled by the advent of the Genomic era, allowing deep knowledge on single or consortia uncultivable species. The establishment and maintenance of symbiosis are complex issues where partners’ fitness determines the evolutionary outcome. Comparative genomics allows to dissect the evolutionary process that begins with host invasion, takes the path from facultative to obligate symbiosis, and ends up in replacement or coexistence with new bacterial symbionts. Whole genomes of several intracellular bacterial symbionts have been sequenced, allowing the comparison among the different evolutionary innovations carried out by these bacteria on their way from free-living to varied stages of integration with their respective hosts. The association and functional interaction of genomes from different species observed during symbiosis, like mutation, recombination, and other genome rearrangements, can be viewed as a source of genetic variation, the fuel for evolution. The action of forces such as natural selection and/or random drift will be responsible for transforming this variation in evolutionary novelties. Host can develop organs to allocate the symbionts and must modify its immune response and its growth rate appropriately to benefit from having one or more symbionts. The bacterial symbiont experiences dramatic changes on its genome, which can be detected by its comparison with free-living relatives. The mechanisms involved in the establishment, maintenance, and evolution of the association can be scrutinized thanks to comparative genomics, whereas systems biology approaches allow us to explore metabolic interdependences among the members of the symbiotic consortium.