[Molecular basis of symbiogenic evolution: from free-living bacteria towards organelles]

Molecular mechanisms of the bacteria evolution are addressed in the context of the theory of symbiogenic origin of eukaryotic cells. In the evolution of symbiotic bacteria two strategies are implemented: (a) combinative, resulted in the formation of symbiotic (sym) gene systems from the genes previously involved in the autonomous life (facultative and ecologically obligatory symbioses); (b) reductive, related to the loss of genes for the autonomous life (genetically obligatory symbioses). Both strategies are based on the increase in genomic plasticity leading: (a) during combinative evolution--to the segregation of sym gene systems into the special plasmids or "islands" (the genome size and complexity increase); (b) during the reductive evolution--to the losses of many metabolic pathways (genome size decreases). These processes are continued during the evolution of mitochondria, hydrogenosomes and plastids in which many genes for the transcription and translation were lost, while the genomes of organelles and nuclei recombined. These reorganizations are related to the peculiarities of the bacteria population dynamics within the symbiotic systems. The bacteria which combine the abilities for symbiotic and autonomous lifestyles are characterized by ecotypic polymorphism (stable coexistence of symbiotic and asymbiotic genotypes). A key role in their evolution is implemented by horizontal transfer of sym genes that in combination with different forms of natural selection (individual, frequency-dependent and group selection) are responsible for the combinative evolution of bacteria genome. In the populations of obligatory symbionts, the genetic drift and group selection dominate that ensure bacteria genome reduction, loss of their biological identity and transformation into organelles.