Rhizobia are soil bacteria that can form nodules in the roots of leguminous plants to initiate a mutualistic relationship, by fixing atmospheric nitrogen for the plant in exchange for carbohydrates. The presence of symbiosis related genes on a Symbiosis Island (SI) or plasmid in the bacterial genome is necessary for a symbiotic association to occur. The SI contains all the genes that enable rhizobia to fix nitrogen and sustain the symbiotic interaction. In the rhizobium genus, Mesorhizobium, the SI may be ejected in a single mutation, under the control of a genetic pathway, as a response to population density that coordinates gene expression. The loss of the SI turns a cooperative strain into a commensal or parasitic strain unable to fix nitrogen. This research investigates if natural Mesorhizobium strains from legume nodules differ in whether they carry the SI. A Polymerase Chain Reaction (PCR) assay was developed, consisting of two sets of primers specific to the SI region, nodB and noeJ, to indicate whether the SI is present or absent. Present research will evaluate if there is heritable variation among different Mesorhizobia strains for the rate of loss of the SI. If there is heritable variation among strains for the rate of loss of the SI, this will show that this trait can evolve by natural selection. If there is no heritable variation for the rate of loss of the SI, we will lack evidence that this trait could evolve by natural selection. To answer these questions, Mesorhizobium will be cultured at high density, bacterial colonies will be chosen, and used in a PCR to observe which strains have lost the SI, and rates of loss can be compared between Mesorhizobium genotypes.
Of the twelve strains sampled, four strains completely lacked the SI, and the other eight contained the SI as confirmed by nodB and noeJ primers. Therefore, we demonstrate natural variation for the presence and absence of the SI among different genotypes in the Mesorhizobia population. These rhizobia are important in agriculture and natural ecosystems, to provide the limiting nutrient nitrogen to the soil. Non-fixing rhizobia that stow away in nodules and use plant resources but do not fix nitrogen can be highly problematic for agricultural production. Knowing the potential for cooperative nitrogen fixers to evolve into uncooperative ones is beneficial in developing strategies to maintain or increase biologically fixed nitrogen, rather than using fertilizer in an agricultural context.