Single-strain inoculation may create spurious correlations between legume fitness and rhizobial fitness

摘要

A recent meta-analysis by Maren Friesen suggests that there is 'widespread fitness alignment' between rhizobia and their legume hosts (Friesen, 2012). The work is motivated by a very important question, namely how is cooperation stabilized between partners in a mutualism? This is a puzzle in the legume—rhizobia partnership because there are potential opportunities for 'cheating' by the rhizobia! symbiont. For example, rather than fixing N_2 for the benefit of the host, rhizobial strains can divert more energy to the synthesis of polyhydroxybutyrate (PHB) (Hahn & Studer, 1986; Cevallos et al, 1996), which enhances their own survival and reproduction (Ratcliff er al, 2008). Similarly, rhizobial strains that interfere with plant ethylene signaling increase their own fitness at the expense of the plant (Ratcliff & Denison, 2009). Given this scope for symbiont defection, why hasn't the mutualism collapsed?