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: nod/nol/noe (encoding for lipo-chito-oligosaccharide Nod factors eliciting the nodule development) and nif/ fix (encoding for nitrogenase enzyme implementing the N2 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 sym 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 sym 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 sym genes to the heterologous genetic backgrounds required for involvement of HGT products into the evolution of mutual interactions.