CONICET | Buscador de Institutos y Recursos Humanos

Despite the importante of mutualism as a key ecological process, its persistence in nature is difficult to explain since the existence of exploitative, cheating partners that could erode the interaction is common.By analogy with the proposed policing strategy stabilizing intraspecific cooperation, host sanctions against non-N2 fixing, cheating symbionts have been proposed as a force stabilizing mutualism in legumeRhizobium symbiosis. Following this proposal, penalizations would include decreased nodular rhizobial viability and/or early nodule senescence in nodules occupied by cheating rhizobia. In this work, we analyse the stability of Rhizobiumlegume symbiosis when non-fixing, cheating strains are present, using an experimental and modeling approach. We used split-root experiments with soy bean plants inoculated with two rhizobial strains, a cooperative, normal N2 fixing strain and a nisogenicnon-fixing, perfect cheating mutant derivative that lacks nitrogen as eactivity but has the same nodulationabilities inoculated tosplit-rootplants. We found no experimental evidence of functioning plant host sanctions to cheater rhizobia based on nodular rhizobia viability and nodule senescence and maturity molecular markers.Based on these experiments, we developed a population dynamic model with and without the inclusion of plant host sanctions. We show that plant populations persist in spite of the presence of cheating rhizobia without the need of incorporating any sanction against the cheater populations in the model, under the realistic assumption that plants can at least get some amount of fixed N2 from the effectively mutualisticr hizobia occupying some nodules. Inclusion of plant sanctions leads to the unrealistic effect of ultimate extinction of cheater strains in soil. Our simulation results are in agreement with increasing experimental evidence and theoretical work showing that mutualisms can persist in presence of cheating partners.