Symbiogenetics and Symbiogenesis: Molecular and Ecological Bases of Integrative Evolution

Symbiogenetics is presented as a discipline aimed to study the combined genetic systems resulted from co-evolution of unrelated organisms. Their research complements the methodology of genetic analysis with the approaches of “genetic synthesis” aimed at characterizing the molecular and ecological factors of integration of heterologous genomes. Hologenomes and symbiogenomes that arise via integrative evolution (symbiogenesis) are dissected, like prokaryotic pangenomes, into the core and accessory parts. In hologenome, the core part, which is constant in composition, is represented by the host genome (nuclear-cytoplasmic system of heredity), and the variable accessory part composed by metagenome of the microbial community which is associated with the host and performs functions useful for it. Mechanisms of symbiogenesis go beyond the factors of evolution of free-living organisms and include: (a) interspecies altruism associated with the refusal of symbionts from autonomous existence, and upon a deep reduction, from the ability to maintain the genome; (b) inheritance by host of symbionts as of “acquired” genetic determinants (pangenesis). Under the impacts of these factors, symbionts can be transformed into cellular organelles that have lost their genetic individuality and sometimes lack genomes. Symbiogenesis is presented as a multi-stage process, including the emergence of: (i) genome-containing prokaryotic cell; (ii) multi-genomic eukaryotic cell; (iii) multicellular eukaryotes as holobionts composed of host organisms and associated microbial communities. Genome-free organelles that have retained the basic reproductive and metabolic functions can be used as models for reconstructing the early stages of cell evolution, including the emergence of cellular genome.