Conflict Over Control in the Legume-Rhizobia Symbiosis

Microbes can dramatically alter the fitness of host organisms, ranging in effect from mutualistic to antagonistic. Across this spectrum of fitness effects the microbial symbiont is predicted to optimize its own fitness benefits either through positive feedback (i.e., mutualism) or exploitation (i.e., antagonism). The legume-rhizobium symbiosis has emerged as a powerful system to study the control mechanisms of hosts and the corresponding subversion of control by symbionts. Rhizobial bacteria are housed in legume root nodules where they provide the costly services of nitrogen fixation in return for host derived carbon. However, variation in symbiont quality and lifestyle strategies can result in fitness conflict between host and symbiont. I used the symbiosis between Lotus japonicus and Mesorhizobium loti to investigate host control over symbionts across the mutualism-antagonism spectrum. For chapter 1 I investigated the effects of host control on symbiont fitness when symbionts vary in their fitness effects. I found that the L. japonicus host can adaptively alter the fitness of symbionts dependent on symbiotic nitrogen fixation. For chapter 2 I examined how varying host investment into symbiosis affects the fitness of hosts and symbionts. I found host control was maintained when host investment into symbiosis was altered. However, at high levels of host investment in symbiosis with a high-quality symbiont I uncovered evidence that host fitness is optimized, while symbiont fitness continues to increase. For chapter 3 I tested the effects of host control and varying levels of host investment on the evolution of symbiont services to hosts. After experimentally evolving rhizobial symbionts of mediocre quality in multiple experiments, the phenotypic data was most consistent with a shift towards antagonistic phenotypes. Together, these results highlight the intense conflict over resources that can lead to the destabilization or breakdown of the mutualism between legumes and rhizobia