Coevolution is predicted to depend on how the genetic diversity of interacting species is geographically structured. Plant-microbe symbioses such as the legume-rhizobium mutualism are ecologically and economically important, but distinct life history and dispersal mechanisms for these macrobial and microbial partners, plus dynamic genome composition in bacteria, present challenges for understanding spatial genetic processes in these systems. Here we study the model rhizobium Ensifer meliloti using a hierarchically-structured sample of 191 strains from 21 sites in the native range and compare its population structure to that of its host plant Medicago truncatula. We find high local genomic variation and minimal isolation by distance across the rhizobium genome, particularly at the two symbiosis elements pSymA and pSymB, which have evolutionary histories and population structures similar to each other and distinct from both the chromosome and the host. While the chromosome displays weak isolation by distance, it is uncorrelated with hosts. Patterns of discordant population structure among elements with the bacterial genome has implications for bacterial adaptation to life in the soil versus symbiosis, while discordant population genetic structure of hosts and microbes might restrict local adaptation of species to each other and give rise to phenotypic mismatches in coevolutionary traits.

Understanding how climate refugia and migration over great distances have facilitated species survival during periods of past climate change is crucial for evaluating contemporary threats to biodiversity. In addition to tracking a changing climate, extant species must face complex, anthropogenically fragmented landscapes. The dominant conifer species in the mesic temperate forests of the Pacific Northwest are split by the arid rain-shadow of the Cascade Range into coastal and interior distributions, with continued debate over the origins of the interior populations. If the Last Glacial Maximum extirpated populations in the interior then postglacial migration across the arid divide would have been necessary to create the current distribution, whereas interior refugial persistence could have locally repopulated the disjunction. These alternative scenarios have significant implications for the postglacial development of the Pacific Northwest mesic forests and the impact of dispersal barriers during periods of climate change. Here we use genotyping-by-sequencing (ddRADseq) and phylogeographical modeling to show that the postglacial expansion of both mountain hemlock and western redcedar consisted largely of long-distance spread inland in the direction of dominant winds, with limited expansion from an interior redcedar refugium. Our results for these two key mesic conifers, along with fossil pollen data, address the longstanding question on the development of the Pacific Northwest mesic forests and contrast with many recent studies emphasizing the role of cryptic refugia in colonizing modern species ranges. Statement of SignificanceUnderstanding whether habitat fragmentation hinders range shifts as species track a changing climate presents a pressing challenge for biologists. Species with disjunct distributions provide a natural laboratory for studying the effects of fragmentation during past periods of climate change. We find that dispersal across a 50-200-km inhospitable barrier characterized the expansion of two conifer species since the last ice age. The importance of migration, and minimal contribution of more local glacial refugia, contrasts with many recent studies emphasizing the role of microrefugia in populating modern species distributions. Our results address a longstanding question on the development of the disjunct mesic conifer forests of the Pacific Northwest and offer new insights into the spatiotemporal patterns of refugial populations and postglacial vegetation development previously unresolved despite decades of paleoecological studies.

Questions: Can resource-resource trade mutualism offer a competitive advantage to plants? If so, what are the conditions that give mutualism an advantage especially with regard to the size of the neighborhood? Hypothesis: We hypothesized that mutualism could offer a competitive advantage if the benefits outweighed the costs. We also hypothesized that this competitive advantage could lead to coexistence between mutualist and non-mutualist strategies within the same population. We also hypothesize that local neighborhood size (the number of competitors at a given moment) would change this response, though the specific direction of change was unclear to us. Method: We created an evolutionary game theoretic model in which a plant could either be a mutualist or non-mutualist that incorporated nutrients freely available to the plant, nutrients obtained only through mutualism with microbes, the cost of producing roots, the cost of trade with microbes, and neighborhood size. We sought ESS solutions as defined by the Nash equilibrium criterion. Key Assumptions: 1) Costs and benefits are fixed for all plants. 2) Freely available nutrients are equally shared between all competing plants in a local neighborhood. 3) Microbially obtained nutrients are shared equally between mutualistic plants in the local neighborhood. Conclusion: We found that mutualism could offer a competitive advantage if the net benefit was positive. Coexistence between mutualist strategies in our model happens because of competition between mutualists over the microbially available nutrient. Coexistence was more likely with greater neighborhood size but at the expense of mutualist fixation.

Free-air CO2 enrichment (FACE) experiments have elucidated how climate change affects plant physiology and production. However, we lack a predictive understanding of how climate change alters interactions between plants and endophytes, critical microbial mediators of plant physiology and ecology. We leveraged the SoyFACE facility to examine how elevated [CO2] affected soybean (Glycine max) leaf endophyte communities in the field. Endophyte community composition changed under elevated [CO2], including a decrease in the abundance of a common endophyte, Methylobacterium sp. Moreover, Methylobacterium abundance was negatively correlated with co-occurring fungal endophytes. We then assessed how Methylobacterium affected the growth of co-occurring endophytic fungi in vitro. Methylobacterium antagonized most co-occurring fungal endophytes in vitro, particularly when it was more established in culture before fungal introduction. Variation in fungal response to Methylobacterium within a single fungal operational taxonomic unit (OTU) was comparable to inter-OTU variation. Finally, fungi isolated from elevated vs. ambient [CO2] plots differed in colony growth and response to Methylobacterium, suggesting that increasing [CO2] may affect fungal traits and interactions within the microbiome. By combining in situ and in vitro studies, we show that elevated [CO2] decreases the abundance of a common bacterial endophyte that interacts strongly with co-occurring fungal endophytes. We suggest that endophyte responses to global climate change will have important but largely unexplored implications for both agricultural and natural systems.