Measured sanctions: Legume hosts detect quantitative variation in rhizobium cooperation and punish accordingly

Abstract

Question: Does severity of punishment vary quantitatively with partner cooperation? Hypothesis: Sanctions against defecting partners may be crucial for the evolutionary persistence of cooperation. Legume sanctions have been demonstrated when rhizobia either fully defect or fully cooperate, but not when they invest at an intermediate level. We predicted that intermediate rates of cooperation would trigger intermediate sanctions. Model system: We varied rhizobium cooperation and its importance to the plant by adjusting N2 concentration, manipulating rhizobia to fix N2 at about 1%, 17%, 33%, 50%, and 100% of their potential, and/or by adding nitrate. Results: Fixation and rhizobium fitness were significantly correlated in a regression model suggesting that sanction strength varies with N2 fixation. Sanction severity was increased by the addition of external nitrate.