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Browsing by Subject "mutualism"

Now showing 1 - 6 of 6
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    Breaking the weakest link: Evolution and ecology of antibiotic tolerance in cross-feeding bacterial communities
    (2019-05) Adamowicz, Elizabeth
    Microbes frequently rely on metabolites excreted by other bacterial species, but little is known about how this cross-feeding influences the effect of antibiotics. We hypothesized that when species rely on each other for essential metabolites, the minimum inhibitory concentration (MIC) for all species will drop to that of the "weakest link"— the species least resistant in monoculture. We tested this hypothesis in an obligate cross-feeding system that was engineered between Escherichia coli, Salmonella enterica, and Methylobacterium extorquens. The effect of tetracycline and ampicillin were tested on both liquid and solid media. In all cases, resistant species were inhibited at significantly lower antibiotic concentrations in the cross-feeding community than in monoculture or a competitive community. However, deviation from the "weakest link" hypothesis was also observed in cross-feeding communities apparently as a result of changes in the timing of growth and cross-protection. Comparable results were also observed in a clinically relevant system involving facultative cross-feeding between Pseudomonas aeruginosa and an anaerobic consortium found in the lungs of cystic fibrosis patients. P. aeruginosa was inhibited by lower concentrations of ampicillin when cross-feeding than when grown in isolation. These results suggest that cross-feeding significantly alters tolerance to antibiotics in a variety of systems.
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    Context-dependence, coexistence, and community structure in microbial cross-feeding mutualisms
    (2022-02) Hammarlund, Sarah
    Mutualisms—reciprocally positive interspecies interactions—are critical for the survival and reproduction of many organisms, from bacteria and fungi to plants and animals. Despite the ubiquity of mutualism, there are many questions that remain underexplored. For example, it is unclear why mutualisms sometimes break down when environmental conditions change. Another open area of research is the ecology and evolution of so- called multiple mutualist communities that contain many interacting species. My research aims to understand how environmental factors alter interactions and stability within mutualisms, to explore how competitors within multiple mutualist communities coexist despite competition, and to identify the selective forces that influence whether mutualists interact with one or multiple partners. I use an experimental system of two species of bacteria that engage in mutualism via cross-feeding—the consumption of nutrients secreted by another species—as well as mathematical and computational modelling. My findings contribute to our understanding of mutualism broadly, and may be useful for efforts to manipulate microbial communities and to protect mutualistic species that are vulnerable to extinction.
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    Data from "Detrimental effects of rhizobial inoculum early in the life of the partridge pea Chamaecrista fasciculata"
    (2018-01-19) Pain, Rachel E; Shaw, Ruth G; Sheth, Seema N; repain@umn.edu; Pain, Rachel E
    Premise of the study: Mutualistic relationships with microbes may aid plants in overcoming environmental stressors, and increase the range of abiotic environments where plants can persist. Rhizobia, nitrogen-fixing bacteria associated with legumes, often confer fitness benefits to their host plants by increasing access to nitrogen in nitrogen-limited soils, but effects of rhizobia on host fitness under other stresses, such as drought, remain unclear. Methods: In this greenhouse study, we varied application of rhizobia (Bradyrhizobium sp.) inoculum and drought to examine whether the fitness benefits of rhizobia to their host, the partridge pea (Chamaecrista fasciculata), would differ between drought and well-watered conditions. Plants were harvested nine weeks after seeds were sown. Key results: Young Chamaecrista fasciculata plants that had been inoculated had lower biomass, leaf relative growth rate, and stem relative growth rate compared to young uninoculated plants in both drought and well-watered environments. Conclusions: Under the conditions of this study, the rhizobial inoculation imposed a net cost to their hosts early in development. Potential reasons for this cost include allocating more carbon to nodule and root development than to above-ground growth and a geographic mismatch between the source populations of host plants and rhizobia. If developing plants incur such costs from rhizobia in nature, they may suffer an early disadvantage relative to other plants, whether conspecifics lacking rhizobia or heterospecifics.
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    The Effect of Competitive and Mutualist Bacteria on the Infection Rate of Generalist Phage N
    (2021) Metzger, Carissa, A
    To better understand the host range of viruses, it is important to understand how interspecies interactions such as mutualism and competition impact how and why viruses are able to infect multiple hosts or just one. In this experiment, lawns of E. coli and S. enterica were plated in either a cooperative environment or a competitive environment and subsequently infected with both a generalist bacteriophage and a specialist bacteriophage. The generalist phage, N, was able to infect both E. coli and S. enterica. T7 was one specialist phage and could only infect E. coli. P22 was the other specialist phage and could only infect S. enterica. In the competitive environment, mathematical modeling predicted that the generalist would perform better than the specialist phage, resulting in a higher yield of the generalist after 48 hours. In the cooperative environment, modeling predicted that the specialist would outperform the generalist phage, resulting in a higher yield of specialist after 48 hours. In the cooperative experiment, the generalist was expected to have a lower infection rate compared to the initial phage dilutions. While our experimental results in both competitive conditions, T7/N and P22/N, were consistent with our hypothesis, the cooperative resulted deviated: in the P22/N data, the specialist phage indeed outperformed the generalist, but the opposite was seen in the T7/N data. However, our data were not statistically significant, and there were many possible sources of error that could have led to these unexpected results, such as measurement or mathematical errors resulting in unbalanced starting phage ratios, N being better at infecting E. coli than S. enterica, or due to contamination of the cultures with bacteria resistant to T7. If after further experimentation and resolution of these errors, these data hold true, this would be extremely interesting for further investigate as they do not match expectations based on modeling or batch culture experimentation.
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    Evolution of energy storage traits in symbiotic rhizobia
    (2018-01) Muller, Katherine
    The mutually-beneficial symbiosis between legume plants and nitrogen-fixing rhizobia involves an inherent conflict-of-interest over how rhizobia allocate the resources they receive from the host plant. In theory, rhizobia could enhance their future fitness by diverting resources from nitrogen-fixation into storage compounds like polyhydroxybutyrate (PHB). Although the conflict-of-interest between PHB accumulation and nitrogen-fixation has been discussed as a driving factor in the evolution of legume-rhizobia interactions, its role in natural populations is unclear. Therefore, this dissertation fills in key empirical gaps between what we know about the functional role of PHB and hypotheses about how natural selection might act on continuous variation in the amount of PHB that rhizobia acquire during symbiosis. The first chapter assesses the extent of heritable phenotypic variation within natural rhizobia populations interacting with soybean (Glycine max) and partridge pea (Chamaecrista fasciculata) and evaluates implications for fitness in the free-living stage after rhizobia are released from nodules. The results from my first chapter show that 1) natural populations of rhizobia contain heritable, quantitative variation in the amount of PHB they accumulate during symbiosis (a prerequisite for evolution by natural selection) and, 2) natural selection on PHB accumulation may be mediated by how rhizobia allocate PHB over time and among life functions, which could vary independently from traits underlying the amount of PHB acquired during symbiosis. The second chapter assesses phenotypic response to selection based on resident rhizobia populations from long-term agricultural plots varying in host (soybean) or non-host (maize) frequency over years. The mean PHB per cell (measured in nodules) was two times higher in rhizobia populations from plots with 5 or 30 years of continuous maize than from plots where soybean was grown in the previous year. An apparent decrease in mean PHB per cell after the first year of soybean following five years of maize supports the hypothesis that low-PHB rhizobia have higher reproduction in nodules, perhaps due to host sanctions against rhizobia that divert more resources to PHB. A model used to interpret the results suggests that, 1) PHB acquired during symbiosis may contribute to fitness variation for several years after the last host crop, and 2) host sanctions against less-beneficial rhizobia may be stronger in the first soybean crop due to a combination of lower initial rhizobia population size and negative frequency-dependent selection during symbiosis. Collectively, these findings provide empirical support for previously unsubstantiated hypotheses about how conflicts-of-interest over resource allocation contribute to the evolution of the legume-rhizobia mutualism and develop a more nuanced framework for future research.
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    Phylogenetics of Cecropieae (Urticaceae) and the evolution of an ant-plant mutualism in Cecropia
    (2017-07) Treiber, Erin
    Mutualisms are common in all ecosystems and can influence ecosystem function in aspects including nutrient cycling and community building. Mutually beneficial interactions between species vary in degrees of specificity from facultative and generalized associations to obligate and specialized associations. Ant-plant mutualisms are especially common in the tropics and are popular for ecological studies of mutualism. Investigating the evolution of tropical ant-plant mutualisms requires a phylogenetic framework. The neotropical ant-plant mutualism involving the large genus Cecropia (Urticaceae) and associated ants, including the genus Azteca, has been the subject of extensive ecological study but has not been examined from a phylogenetic perspective. Large woody plant genera have often been difficult to treat taxonomically and resolve phylogenetically. It is unknown whether this is due to variation in traditional DNA markers or if other factors are involved. Next generation sequencing techniques can provide more data than direct methods that improve phylogenetic resolution and provide opportunity to infer historical introgression that could also influence phylogenetic resolution. The goal of this dissertation is to investigate evolutionary relationships in Cecropia and its closest relatives and to examine the evolutionary history of the mutualism. Relationships between genera in the tribe Cecropieae (Urticaceae), including Cecropia, Coussapoa, Musanga, Myrianthus, and Pourouma, were unknown but are necessary to investigate the evolutionary history of the Cecropia-ant mutualism. Chapter 1 used molecular phylogenetics to infer relationships between genera in Cecropieae and investigates the influence of phylogenetic resolution in Cecropia on reconstructing the ancestry of myrmecophytism. Bayesian phylogenetic analysis of the NADH dehydrogenase (ndhF) chloroplast gene region, the 26S region of nuclear ribosomal DNA, and an exon-primed intron-crossing DNA region supported non-myrmecophytic African Musanga within a paraphyletic Cecropia. Neotropical Pourouma and Coussapoa were supported as sister taxa with African Myrianthus as their closest relative. Although it was uncertain whether myrmecophytism was the ancestral condition of Cecropia, a close relationship between non-myrmecophytic Cecropia sciadophylla and Musanga suggests that the loss of ant associations did not accompany African colonization by Musanga. In Chapter 2, restriction site associated DNA (RAD) sequencing was used to infer relationships among myrmecophytic and non-myrmecophytic Cecropia species. RAD sequence data resolved and supported species level relationships beyond what could be inferred from direct sequencing. The D-statistic to test for introgression among Cecropia species was used to examine whether hybridization might account for some of the difficulty associated with diagnosing species in the genus. Most Cecropia species sampled were not deeply diverged genetically but a non-myrmecophytic clade included lineages that could be considerably older than most of the ant-associated species. Results of ABBA BABA tests could be interpreted as evidence of recent introgression among closely related myrmecophytic species. However, test results implying geographically implausible introgression between neotropical C. sciadophylla and afrotropical Musanga suggest that the D-statistic is sensitive to the extent of genetic divergence among clades and may yield type I error in the case of deeply diverged clades. Evidence from geographically widespread and morophologically hetergenous C. obtusifolia and C. angustifolia suggests that current synonymy lumps together genetically dissimilar lineages and that future taxonomic revision should consider splitting. Chapter 3 investigated the origin and evolutionary history of myrmecophytism in Cecropia sensu lato (including Musanga). The most highly supported phylogeny for Cecropia was used in ancestral state reconstructions of ant association and the myrmecophytic traits of domatia (nest cavitiy) and trichilia (food bodies) to investigate the evolutionary history of the mutualism. Although it was unknown whether the common ancestor of Cecropia was myremecophytic, the deepest split in the clade revealed ecological differences between the two oldest lineages of Cecropia sensu lato. The clade including C. sciadophylla and Musanga more likely had a non-myrmecophytic ancestor while myrmecophytism was most likely the ancestral state of the more species-rich Cecropia sensu stricto. Trichilia were associated with the origin and loss of ant associations where as domatia where not. Cecropia is distributed across a broad range of elevation and the absence of ant associations with high montane species was associated with the evolutionary loss of trichilia in two independent cases. A comparative analysis showed that gains and losses of myrmecophytism in Cecropia were correlated with the presence or absence of trichillia and domatia. Ant associations were more dependent on the presence of trichilia than on domatia. A resolved and highly supported phylogeny of over half the genus may be used to inform future ecological and evolutionary studies of the Cecropia-ant mutualism. A phylogenetic framework for Cecropia will allow for studies to take relatedness into account when comparing morphologogical traits of species. RADseq provided the data needed to begin to resolve relationships within Cecropia and may be what is necessary in other unresolved tropical tree genera. This will also allow for mutualism to be studied in a phylogenetic context where it previously was not possible.