UC Riverside

Understanding the drivers of variation in symbiont quality is a fundamental objective in the study of mutualisms. Eukaryotic hosts express control traits that can selectively favor beneficial symbionts over ineffective genotypes, but bacterial symbionts range widely in beneficial quality. Evolutionary instability in symbiotic function and/or context dependency in the expression of symbiotic traits are predicted to contribute to this variation.

The Acmispon-Bradyrhizobium mutualism is a model system for studying variation in symbiotic traits. Acmispon hosts sanction ineffective symbionts, yet Bradyrhizobium naturally vary in symbiotic function. However, the incidence, distribution, and evolution of symbiotic quality in Bradyrhizobium from natural habitats remains unclear. I investigated the evolution and spatial distribution of Bradyrhizobium symbiotic effectiveness across a metapopulation of A. strigosus hosts. Symbiotic quality was evolutionarily unstable, consistent with the repeated evolution of non-nitrogen fixing Bradyrhizobium, and suggests that the loss of nitrogen fixation may be a common process.

To examine if symbiotic ineffectiveness was expressed as a maladapted, context dependent outcome, I quantified fitness and fitness-effects of divers Bradyrhizobium isolates on sympatric and allopatric Acmispon hosts. Several isolates were found to be symbiotically ineffective and rhizobial fitness proxies uncovered evidence of rhizobial exploitation. This data suggests that host exploitation also maintains ineffective rhizobia, and thus overall variation in symbiont quality in natural populations.

Symbiotic quality can also vary due to biotic environment, and although rhizobia are best known for symbiotic function, the majority of rhizobia are non-symbiotic. To test if non-symbiotic conspecifics effect the Acmispon-Bradyrhizobium mutualism, I coinoculated hosts with mixtures of symbiotic and non-symbiotic Bradyrhizobium. In most cases, the presence of non-symbiotic Bradyrhizobium reduced host and symbiont performance and data suggests this occurs via competitive interactions at the root-soil interface.