Standing genetic variation in the effect of a locus negotiating mutualism outcomes in wild rhizobia on their natural host plant

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Camille Wendlandt, Crop & Soil Sciences, Washington State University, Vancouver, WA, Miles Roberts, Kyle Nguyen, Zoie Lopez and Emily Helliwell, Washington State University, Vancouver, Marion Graham and Paul Price, Eastern Michigan University, Eric von Wettberg, University of Vermont, Maren Friesen, Department of Plant Pathology, Washington State University, Pullman, WA, Joel Griffitts, Microbiology and Molecular Biology, Brigham Young University, Provo, UT, Stephanie Porter, School of Biological Sciences, Washington State University, Vancouver, Vancouver, WA

Background/Question/Methods
Mutualism between hosts and symbiotic microbes is ubiquitous, but we know little about the genetics underlying fitness in wild mutualist populations. Variation at loci that determine mutualist fitness provides raw material upon which coevolutionary selection acts and sets the dynamics and pace of coevolution. In the mutualism between nitrogen-fixing rhizobium bacteria and leguminous plants, the plasmid-borne rhizobial locus, hrrP, can determine fitness outcomes for each partner. Rhizobia lacking hrrP can typically infect hosts and respond to host cues to differentiate into a nitrogen-fixing form. However, rhizobia bearing the B800 hrrP allele can degrade host cues, depressing host fitness and enhancing symbiont fitness. The large effect size of the B800 hrrP allele suggests that hrrP could drive substantial variation in mutualist fitness. To investigate the incidence and effect size of hrrP alleles in wild rhizobium populations, we measure: 1) the prevalence of the hrrP locus in Ensifer medicae rhizobia, 2) the symbiotic phenotypes of naturally occurring hrrP+ and hrrP- E. medicae, and 3) the range of effect sizes of hrrP alleles in contrasting environments for novel isogenic hrrP knock out mutants.

Results/Conclusions
Wild rhizobia vary in the presence/absence of hrrP, and in the effects of individual hrrP alleles on host fitness. hrrP occurs in 12.5% of E. medicae from a wild collection of 400 strains, with hrrP+ strains present in 56% of populations. Corroborating previous work, hrrP generally enhances symbiont fitness at the expense of host fitness: hrrP+ strains provide 8% lower plant shoot mass, but produce 63% more root nodules, compared to hrrP- strains. However, hrrP alleles show striking variation, ranging from negative to positive effects on host and symbiont fitness, when comparing hrrP knock-out mutants to isogenic wild strains. These effect sizes differ by year of experiment and plant host genotype. The variable effects of a locus determining mutualist fitness could result from arms race dynamics, in which hosts evolve to neutralize the effect of alleles conferring exploitation in a symbiont, and vice versa. The context-dependent effects of hrrP could arise if host and symbiont fitness depend on a balance between hrrP activity (degradation of host cues) and host production of those cues; this type of physiological dynamic could drive highly context-dependent fitness effects in mutualism. Overall, the large and variable effects of a locus for fitness in mutualism presents a challenge to attempts to identify the genetic basis of cooperation via statistical association between genotype and phenotype.