Symbiotic consortia: a model

Our understanding of prokaryote-eukaryote symbioses as a source of evolutionary innovation has rapidly increased by the advent of genomics, making possible the biological study of uncultivable endosymbionts. Genomics is allowing the dissection of the evolutionary process that starts with host invasion, goes from facultative to obligate symbiosis, and ends up in replacement or coexistence with new symbionts. Moreover, it has provided important clues on the mechanisms driving the genome reduction process, the functions that are retained by the endosymbionts, the role played by the host, and the factors that may determine whether the association will become parasitic or mutualistic. I present a model to describe the evolutionary history of animal-bacteria obligatory mutualistic symbiosis. Dispensability of genes and genetic isolation are key factors in the reduction process of these bacterial genomes. Major steps in such genome reductive evolution, leading towards primary endosimbiosis, and the possibility of complementation or replacement by a secondary symbiont are also shown. The mechanisms involved in the establishment, maintenance and evolution of the association can be scrutinized thanks to comparative genomics, while systems biology approaches allow us to explore metabolic interdependences among the members of the symbiotic consortium. Finally, the comparative analyses of reduced symbiont genomes are of relevance in the field of synthetic biology. In-silico studies of metabolic and protein-protein interaction networks performed in simple but natural minimal cells are useful to try to define hypothetical cells with even shorter genomes.

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