SYMBIOSIS AND EVOLUTION

Symbiosis is a widespread biological phenomenon which plays an important role in the ecology and the physiology of organisms. The associated partners are not only juxtaposed but interact each other. The symbiotes both supply the host with growth factors and modify its metabolism and its gene expression. It is proposed that the different modalities of symbiosis can be classified into 5 main types. After a long coevolution the partners often became dependant from each other. When the symbiote is completely integrated and controlled by the host it can be considered as a cytoplasmic organelle. Therefore, symbiosis could be the most sophisticated mechanism of gene predation in the eukaryotes and the symbiote assimilated to a kit of new genes acquired by the host. This probably represents the most efficient mechanism of innovation and evolution of organisms. According to the endosymbiotic theory the eukaryote cell was formed by successive symbioses with prokaryote cells having subsequently evolved to organelles. The coevolution of symbiote and host genomes is depicted by gene transfer and codependance. Is symbiosis also implicated in organism speciation ? It is difficult to obtain decisive proofs but such a conclusion is supported by a lot of considerations. In fact, symbiosis is the formation of a new biological unit, the symbiocosm ; the consequence is a general modification of the phenotype of the partners (morphology, biochemistry and behavior) and a better adaptation to the environment. Furthermore, symbiosis is capable to generate incompatibility between different strains, and to favor reproductive isolation. For all these reasons we consider symbiosis as a very important mechanism of biological innovation, and it has been retained by natural selection in numerous species. This theory is not incompatible with the neodarwinian synthetic theory, but it needs that we accept a greater diversity concerning the mechanisms of genetic novelty, which cannot be restricted to genic mutations and chromosomal rearrangements.