Mutualisms between plants and nitrogen fixer bacteria are an important ecological and economic interaction. The bacteria are sheltered in root nodules where they perform nitrogen fixation (NF), in exchange for carbon. NF is a costly process and bacteria may cheat their plant partners by fixing less nitrogen. Many studies focus on how the plant-bacteria interaction persists despite cheaters. However, little is known about how the bacteria genes involved in this interaction—related to nodulation and NF—became stable. These genes can bring benefits to the bacteria, like nitrogen itself and carbon from the plant. They can be in different plasmids, in the same plasmid, or even incorporated into the bacterial chromosome. When these genes are in plasmids, they can be transmitted by horizontal genes transfer (HGT, or sexual reproduction). In HGT, the bacteria pays a cost due to plasmid reproduction. Plasmids can also spread by vertical transmission when the host bacteria reproduce. Thus, bacteria-plasmid interactions may be characterized as an ecological interaction, varying from antagonistic to mutualistic. We studied the importance of HGT to the interaction between the bacteria and plasmids involved in NF and nodulation. We made a theoretical model to understand how these genes are maintained in bacteria.
Results/Conclusions
When we consider only one plasmid interacting with the bacteria, it is a stable interaction if the benefits brought by the plasmid genes and the HGT rate are high enough. With two plasmids, we observe the least costly plasmid dominating in the bacteria population. However, if the HGT of the costliest plasmids is high enough, the population of bacteria will have both plasmids coexisting. If both plasmids present high HGT rates, the bacteria population is a mix of the two-plasmid bacteria, bacteria with the least costly plasmid, and plasmid-free bacteria. The incorporation of the least costly plasmid into chromosome (i.e., no HGT) also keeps the genes of both plasmids in the population. When both genes are incorporated into the chromosome, then bacteria with the least costly gene dominates the population. We believe HGT plays an important role in the ecological interaction of plasmids and bacteria. We believe that NF genes, likely the costliest ones, coexist with genes involved in the nodulation that are either on plasmids or in the chromosome. For its persistence in bacteria, the NF genes must have high HGT. Nitrogen availability and spatial distribution may change some of these results.