Bacterial genome evolution in superspecies systems: An approach to the reconstruction of symbiogenesis processes.

Bacteria form a broad spectrum of symbioses with eukaryotes. This permits reconstruction of the symbiogenesis processes providing the transformation of free-living microorganisms into cellular organelles. In ecologically (conditionally) obligate symbioses, an increase in the size and complexity of the bacterial genome structure was observed. This was associated with segregation of the regions controlling symbiosis into gene clusters, islands, and plasmids. In genetically (strictly) obligate symbioses, a reduction of 'nonsymbiotic' regions of microbial genome occurs, which could begin from genes encoding metabolic and regulatory functions. It is extended towards genes encoding template processes. Conditionally obligate symbioses are characterised by the activation of horizontal gene transfer between various forms of microsymbionts, while for strictly obligate intracellular symbioses an activation of endo-symbiotic gene transfer between microsymbionts and their hosts was detected. The latter is responsible for bacterial transition from the functional (based on gene cross-regulation) to structural (based on recombination) genetic integration with hosts, which later could be followed by the complete assimilation of microbial genomes. In α-proteobacteria this evolutionary pathway could result in the formation of cellular organelles that are deficient in their own genomes but capable of preserving proteomic and cytological traits as a result of the gene-product import synthesized in cytosol (hydrogenosomes and mitosomes). The symbiogenic evolution of cyanobacteria could result in the loss of the plasmids generated from them, while the host maintains a significant part of their genome in nuclear chromosomes. [ABSTRACT FROM AUTHOR]