Impact of Horizontal Gene Transfer on Evolution of Root Nodule Bacteria Directed by Host Plant
Horizontal gene transfer (HGT) plays a key role in the evolution of root nodule bacteria (rhizobia) that provide the nitrogen input to terrestrial ecosystems. HGT in these bacteria pertains the symbiotic (sym) genes: <i>nod/nol/noe </i>(encoding for lipo-chito-oligosaccharide Nod factors eliciting the nodule development) and <i>nif/ fix </i>(encoding for nitrogenase enzyme implementing the N<sub>2</sub> fixation). The crucial role of HGT in the rhizobia evolution is evident at two levels: population (microevolutionary) and phylogenetic (macroevolutionary). At the first level, HGT is responsible for the "gain-and-loss" evolution ensuring a maximal fitness of "ecotypically polymorphous" (composed of symbiotically competent and asymbiotic strains) rhizobia populations circulating in plant-soil ecosystems. At the second level, HGT is responsible for the broad distribution of <i>sym </i>genes among the gram-negative bacteria recruited from the soil and plant-associated communities into symbiotic interactions with legumes. The high impact of HGT on the rhizobia evolution are combined with low frequencies of <i>sym </i>gene transfer, which cannot be detected using the standard laboratory technique. Operation of the symbiosis-specific microevolutionary factors, primarily of frequency-dependent selection induced during the competition of bacteria for nodulation ensures an anchoring of rarely generated recombinants in the plant-associated microbial communities. The variable (individual, group) selective pressures induced by partners' feedbacks is responsible for the adaptation of <i>sym </i>genes to the heterologous genetic backgrounds required for involvement of HGT products into the evolution of mutual interactions.