[Symbiogenesis as a Model for Reconstructing the Early Stages of Genome Evolution]

Symbiogenic evolution, which involves transformations of bacteria into the cellular organelles, is represented as a model for reconstructing the early stages of genome evolution, including the origin of DNA genomes from RNA genomes and the emergence of template processes on the basis of self-replicating molecular complexes in the ancestral metabolic systems. The antiquity of RNA genomes is supported by an increased evolutionary stability of ribosomal protein synthesis (translation) with respect to the DNA-dependent template processes (replication, transcription, recombination, and reparation). This stability is demonstrated by analysis of the deeply reduced genomes of symbiotic bacteria and cellular organelles as well as the "minimal" genomes which are common to phylogenetically diverse organisms. Higher evolutionary conservation of template biosynthetic processes with respect to step processes determining the metabolism and development in cells does not support the hypothesis about emergence ofgenomes within the ancestral cellular metabolic systems which are thought to be of abiogenic origin, instead suggesting dualistic origin of life on Earth. We suppose that the genome-free organelles of some eukaryotes (mitosomes, many hydrogenosomes, and some plastids) represent the products of reversion of symbiotic bacteria into ancestral forms which implemented their basic cellular functions using the informational macromolecules of exogenic origin. In the framework of this hypothesis the eukaryotic cells functioning based on the massive transfer of gene products (RNAs, proteins) from cytosol to organelles may represent the analogs of ancestral biocenoses that possessed integral hereditary systems (metagenomes).