Dr. Liana Burghardt Assistant Professor, Plant Sciences Department Pennsylvania State University Evolving together, evolving apart: Measuring the fitness of rhizobial bacteria in and out of symbiosis with leguminous plants The nitrogen-fixing symbiosis between legumes and rhizobia represents fertile ground to study the evolution and ecology of beneficial plant-microbe interactions. In facultative relationships where both partners can live independently, soil selection patterns can influence the process of host adaptation and vice versa. This talk will describe a twist on an ‘evolve and resequence’ methodology we developed to measure the relative success of scores of co-existing rhizobia isolates in host and nonhost environments. Our team of researchers has measured Sinorhizobium strain fitness in nodules of Medicago legume hosts and as free-living saprophytes in the soil. By synthesizing results from multiple experiments, we examine patterns of rhizobial fitness correlations within and between host and non-host environments. We found 1) only between host species do we observe negative fitness correlations (evolutionary constraint), 2) that in soil mesocosms, temperature and salinity influence isolate-level selection more strongly than other soil characteristics, and 3) that adaptation to nonhost soil environments is unlikely to constrain or undermine rhizobial adaptation to host environments. Our results suggest it may be possible to leverage host genetic variation to manipulate rhizobial communities and optimize rhizobial fitness without undermining the soil survival of rhizobia and that co-occurring Medicago species may function to maintain rhizobial diversity in soils.
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