Genetic and molecular basis of symbiotic adaptations

Abstract

We examined the population and molecular mechanisms of symbiotic adaptations contributing to the structural, functional, and genetic integration of nonrelated organisms. These adaptations are determined by group selection processes operating at the levels of integral super-species systems (holobionts) and their internal microbial communities. This type of selection is responsible for an increase in symbiosis integrity, which is dependent at the early stages of partners’ coevolution on their signaling interactions (cross-regulation of genes) and at the late stages on a allocation of hereditary material between partners (endosymbiotic gene transfer). The resulting increase in the adaptive potential of symbiotic systems is attributed to (i) intra- and interspecies partners’ altruism as a consequence of their mutualistic interactions; and (ii) the inheritance of microsymbionts by hosts as beneficial genetic determinants acquired from the environment in the course of adaptive coevolution. The first mechanism is attributed to the involvement of the host in cooperative adaptations arising within microbial populations and communities in the course of the inoculation and colonization of symbiotic niches. The second mechanism is attributed to realization of the pangenesis hypothesis, proposed by Ch. Darwin as a possible strategy for direct adaptation of organisms to the environment—an alternative to natural selection. In the course of symbiogenesis, this adaptation is implemented on the basis of ontogenetic programs that ensure transitions from an unstable (pseudo-vertical) transmission of facultative symbionts in host progenies towards a stable (transovarian, cytoplasmic) inheritance of obligatory symbionts and cellular organelles.