[Co-evolution of partners and the integrity of symbiotic systems]

Symbioses are very suitable models for studying the integrity of biosystems which characterizes their structural/functional organization enabling the partners to respond adequately to the environmental changes. Analysis of different forms of plant-microbe and animal-microbe symbiosis suggests that a qualitative increase of its integrity occurs under the facultative and ecologically obligatory interactions and is culminated under the genetically obligatory interactions. By use of mathematical models, we demonstrate that the functional integrity of N2-fixing legume-rhizobia symbiosis (concordance of changes in partners' genotypic frequencies induced by environmental fluctuations) correlates to its ecological efficiency which increases under force of natural selection. It results in the tight partners' regulatory feedbacks leading to their genetic integration manifested in the establishment of "symbiogenome". The genetic integrity of symbiosis determines its high evolutionary potential based on: a) epigenetic inheritance of symbiotic traits by hosts which may occur in the form of vertical transmission of either microsymbionts themselves or genes obtained from them; b) interspecies altruism interactions related to the positive partners' feedbacks which determine the ecological efficiency of mutualistic interactions. Realization of this potential results in the deep genetic integration of initially independent partners including their fusions into the novel integral organisms.