Combining GWAS and Population Genomic Analyses to Characterize Coevolution in a Legume-rhizobia Symbiosis

The mutualism between legumes and rhizobia is clearly the product of past coevolution. However, the nature of ongoing evolution between these partners is less clear. To characterize the nature of recent coevolution between legumes and rhizobia, we used population genomic analysis to characterize selection on functionally annotated symbiosis genes as well as on symbiosis gene candidates identified through a two-species association analysis. For the association analysis, we inoculated each of 202 accessions of the legume host Medicago truncatula with a community of 88 Sinorhizobia (Ensifer) meliloti strains. Multistrain inoculation, which better reflects the ecological reality of rhizobial selection in nature than single-strain inoculation, allows strains to compete for nodulation opportunities and host resources and for hosts to preferentially form nodules and provide resources to some strains. We found extensive host by symbiont, that is, genotype-by-genotype, effects on rhizobial fitness and some annotated rhizobial genes bear signatures of recent positive selection. However, neither genes responsible for this variation nor annotated host symbiosis genes are enriched for signatures of either positive or balancing selection. This result suggests that stabilizing selection dominates selection acting on symbiotic traits and that variation in these traits is under mutation-selection balance. Consistent with the lack of positive selection acting on host genes, we found that among-host variation in growth was similar whether plants were grown with rhizobia or N-fertilizer, suggesting that the symbiosis may not be a major driver of variation in plant growth in multistrain contexts

An urban-rural spotlight: evolution at small spatial scales among urban and rural populations of common ragweed

These files include data from our germination and greenhouse experiments. For the germination experiment, we recorded days to germination as well as germinants per cell, which were used to calculate percent germination. For the greenhouse experiment, we measured several phenological (initiation of reproduction, male flowering time, female flowering time) and morphological traits (height and leaf dissection). We selected these traits as they are known to be ecologically important, vary within and among populations of A. artemisiifolia, and that trait variation has a genetic basis. Initiation of reproduction was scored as the date on which a reproductive bud at the apical meristem first appeared. Male flowering time was scored as the number of days from transplanting to first observed open anther. Female flowering time was scored as the number of days from transplanting to the first visible stigma. Plant height was measured during week 7, 9, 11, and 13. After seven weeks, the third fully expanded leaf of each plant was collected to measure leaf dissection index. Leaf area and perimeter were calculated using ImageJ, and leaf dissection index was calculated by dividing leaf perimeter by twice the square root of the product of leaf area and π, or the formula: DI = Perimeter / (2√Area x π).Urbanization produces similar environmental changes across cities relative to their neighboring rural environments. However, there may be high environmental heterogeneity across an urban-rural gradient. Previous research in Minneapolis, MN, USA, found mixed evidence that urban and rural plant populations of common ragweed have locally adapted, and that urban populations exhibit greater among-population divergence in ecologically-important traits. To investigate whether there are parallel patterns of urban-rural trait divergence across different urban areas, we examined trait variation across an urban-rural gradient in a second city, St. Louis, MO, USA. We used growth chamber and greenhouse common environments to investigate variation in six traits within and among 16 populations of common ragweed (eight from each urban and rural area). Urban and rural plants diverged significantly in three of five traits, with rural plants having lower percent germination, greater height and lower leaf dissection index. We also found greater variance in plant height among urban compared to rural populations, potentially driven by heterogeneity in human management practices on urban populations. Patterns of urban-rural trait divergence (e.g. in flowering time) differ substantially from those found previously in the reciprocal transplant experiment in Minneapolis, contradicting the hypothesis of parallel evolution across different metropolitan areas. The results of this study suggest that there is considerable population variation in ecologically-important traits, but that urban populations do not consistently differ from neighboring rural populations.EEB Research Award (University of Minnesota)University of Minnesota Undergraduate Research Opportunities ProgramNSF PGRP grant #185674