Fitness alignment and conflict in the mutualism between plants and nitrogen-fixing soil symbionts
Mutualistic symbionts and environmental conditions alter the expression of plant traits. However, few studies investigate whether evolution of both plant and symbiont populations could contribute to trait evolution or how environmental quality could impact this process. We grew legume lineages in the absence or presence of lineages of nitrogen-fixing symbiotic rhizobium bacteria in two environmental contexts. This allowed us to quantify the effects of symbiotic state, genetic variation, and environmental quality to the expression of commonly measured ecophysiological plant traits. We also examined genotypic selection gradients to measure host-symbiont fitness conflict or alignment over ecophysiological trait values in high vs. low-quality environments.
The presence of rhizobia strongly affected trait expression, with especially large effects in a lower quality environment. We found genetic variation among plant lineages for all focal traits and among rhizobium strains for specific leaf area. Natural selection for plant lineages that flower early but rhizobium lineages that cause plants to flower late suggests hosts and symbionts experience fitness conflict over trait expression and this conflict is more severe in a lower quality habitat. Our results suggest symbiont co-determination of host traits that are not directly related to mutualistic resource exchange may be common and strong. Thus, understanding the evolution of commonly measured ecophysiological traits will be incomplete without explicitly considering the role of symbionts.