Symbiosis and Evolution: At the Origin of the Eukaryotic Cell
Résumé
The cell of eukaryotic organisms (animals, plants, fungi) differs from that of prokaryotic organisms (Archaea and Bacteria) by the presence of several specialized organelles, such as: the nucleus (containing the genetic information of the cell), the mitochondria (site of cellular respiration), or the chloroplast (site of photosynthesis in plants). The existence and organization of mitochondrial and chloroplast DNA, as well as their biochemistry and some structural traits, have led to their being considered as ancient bacteria integrated into a host cell by an endosymbiosis process. One possible hypothesis would be that current eukaryotes would descend from an archaeal ancestor who acquired a proteobacteria, the present mitochondria. Once this step was established, some cells would have incorporated cyanobacteria that are the origin of the chloroplast. At the same time, they have acquired the ability to carry out photosynthesis, and thus an autotrophic metabolism, a particularity of plants. Throughout the process, gene transfer phenomena between symbionts, the taking over of the coding of some organelle proteins by the nucleus and the relocation of gene products into the organelles have closely integrated these prokaryotes within the host cell. The phenomenon of endosymbiosis is therefore very largely responsible for the biodiversity of eukaryotes that appeared during evolution. Thus, photosynthesis has developed in a wide variety of organisms: red and green algae, green plants through primary endosymbiosis, brown algae and many other organisms through secondary or tertiary endosymbiosis.