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Browsing by Subject "Mutualism"

Now showing 1 - 6 of 6
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    The Evolution of Symbiosis in Communities
    (2015-11) Nelson, Paul
    All organisms host a menagerie of symbionts. While harmful pathogens have historically held the attention of researchers, recent technological advances have revealed a cornucopia of benign, and even beneficial, symbionts. Observations that most organisms are party to a wide variety of harmless symbionts are at odds with theory that suggests that infections by multiple symbionts should lead to the evolution of harmful pathogens. Current theory regarding the evolution of symbionts is predicated on the assumption that symbionts receive a reproductive payoff for harming their hosts. Because harming the host, or virulence, indirectly decreases symbiont infection duration, increased symbiont reproduction comes at a cost and leads to a tradeoff. A consequence of this tradeoff is that when multiple symbionts infect the same host the most virulent symbiont receives the highest reproductive payoff while all symbionts suffer decreased infection duration. Consequently, multiple infections are predicted to select for higher virulence, a prediction that runs counter to observation of the plethora of relatively harmless symbionts observed co-infecting most organisms. The three chapters of this thesis seek to bring theory in line with observations of the commonality of co-infecting commensals. The first chapter of this thesis lays out a mathematical model that uses the virulence tradeoff hypothesis to show that multiple infections do not necessarily lead to increased virulence. The second chapter extends the model developed in the first chapter to show that symbiont defense of the host can lead to the evolution of lower virulence. Finally, the third chapter examines ¬genetic variation in virulence and inhibition between symbiont species for fungal symbionts isolated from two populations of maize. Together, this work furthers our understanding of how symbionts evolve in communities and is an important step toward resolving the paradox of ubiquitous benign symbionts.
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    From pattern to process: ecology and evolution of host specificity in the fig-pollinator mutualism.
    (2011-09) Moe, Annika M.
    One of the greatest challenges in the study of coevolution, indeed, for biology in general, is to understand how evolutionary and ecological processes shape patterns in nature. Ecologists routinely observe patterns of association among organisms, such as parasites infecting hosts or insects pollinating flowers and systematists routinely infer patterns of phylogenetic relationship. Such patterns invite explanation and suggest hypotheses about the evolutionary process, but it is difficult to investigate contemporary processes, including natural selection and, of course, impossible to directly observe historical processes. Observation of patterns in various ecological contexts, inference of phylogenetic patterns, model and simulation of processes, and direct experimentation aim to test specific predictions about the role of ecology in shaping evolutionary trajectories, and evolutionary processes in shaping ecological associations. The fig-wasp pollinator mutualism provides a unique opportunity to examine fundamental processes of coevolution, namely, reciprocal adaptation where interacting partners are the agents of selection. Because pollinating wasp reproduction is directly linked to host plant reproduction, it is possible to estimate the fitness consequences of interaction for both partners simultaneously. By manipulation of interacting individuals and species, or by examination of natural variation within and among populations, it may be possible to estimate the strength and direction of selection on each mutualistic partner. This work employs molecular genetic patterns, ecological observations, and direct experimentation to investigate host specificity in Ceratosolen (Agaonidae, Hymenoptera) pollinators of Ficus subgenus Sycomorus (Moraceae) and potential processes affecting the origin and evolution of species diversity in this system. The first chapter examines genetic variation in Ceratosolen pollinators of widespread Ficus across the geographic range of several host species. Deep mitochondrial DNA sequence divergence between host-specific populations distributed across Wallacea suggests host conservatism during ancient range expansion and subsequent isolation by distance. Geographic patterns of sequence divergence and host association are more consistent with a model of allopatric speciation than speciation by host switching. The second chapter investigates pollinator host choice by morphotyping and DNA barcoding of floral visitors in a community of closely related and sympatric fig species. Host specificity was very high, but rare pollinator sharing among sympatric fig species was observed at a rate of 1-2%. Even such rare events could be evolutionarily significant and pose challenges for species delimitation. The third chapter examines fitness consequences of pollinator sharing by experiment. A new method of manipulating fig pollinators investigated the reproductive consequences of intra- and interspecific pollinator visitation for both mutualistic partners. When pollinators were introduced to a novel host species, hybrid seed set was comparable to results of conspecific crosses. Hybrids germinated, established, and grew at rates comparable to non-hybrids. Pollinator fitness, however, was compromised after oviposition in the novel host. Although heterospecific pollinators induced gall formation, offspring did not develop to maturity in the new host. Microsatellite genotypes of a New Guinea fig community indicated a substantial but not absolute barrier to gene flow among sympatric species. That hybrids constituted fewer than 2% of individuals in populations may be explained by selection against pollinator host switching in this system. Collectively, these studies suggest that the extreme species-specificity of associations between Ceratosolen pollinators and Sycomorus figs is maintained by the fitness cost of colonizing new hosts. At the same time, hybridization resulting from rare instances of pollinator sharing in even the most extremely specialized of pollination mutualisms has the potential to influence diversification and coevolution.
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    Population genomics of the legume symbionts Sinorhizobium meliloti and S. medicae
    (2013-11) Epstein, Brendan
    The nitrogen-fixing mutualism between legumes and rhizobia is ecologically and agriculturally important and is a model for the molecular genetics of plant microbe interactions and the evolution of mutualism. The goal of this research was to investigate the evolutionary forces shaping genetic diversity in two species of rhizobia, Sinorhizobium meliloti and S. medicae by integrating population genetic tools, experimental evolution, and whole-genome sequencing. In Chapter 1, I characterize the diversity and divergence of S. meliloti and S. medicae and ask how selection and horizontal gene transfer (HGT) have shaped nucleotide variation. I find limited evidence for HGT between S. meliloti and S. medicae, indicating that recombination with closely related species does not have much impact on nucleotide diversity in Sinorhizobium spp. and does not prevent species from diverging. I also find that the targets of strong positive selection are different in the two species, suggesting that S. meliloti and S. medicae may be subject to different selective pressures in nature. The goal of Chapter 2 was to examine gene content and copy number variation. While I find that S. meliloti and S. medicae both have extensive variation in content and copy number, most of this variation seems to be deleterious. This suggests that the large size of bacterial pangenomes is due, in part, to many short-lived, deleterious gains and losses of genes rather than adaptation. Finally, in Chapter 3, I use experimental evolution to tests for costs of mutualism. I do not find clear evidence of costs, but I do identify a mutation in the purM gene that may affect host range. Overall, this contributes to our understanding of both the evolution of rhizobia, and the evolutionary forces shaping variation in prokaryotes.
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    Resource hoarding facilitates cheating in the legume-rhizobia symbiosis and bet-hedging in the soil.
    (2010-06) Ratcliff, William C.
    The carbon that rhizobia in root nodules receive from their host powers both reproduction and the synthesis of the storage polyester poly3-hydroxybutyrate (PHB), as well as N2 fixation, which mainly benefits the host. Rhizobia escaping nodules can use stored PHB to survive starvation and reproduce up to 3-fold, but PHB synthesis is energetically expensive and trades-off with N2 fixation. As a result, PHB synthesis is a central mechanism in the evolution of conflict between rhizobia and legumes, and should be included in estimates of rhizobial fitness. Some rhizobia have evolved sophisticated mechanisms to increase PHB accumulation, such as the production of rhizobitoxine, a chemical inhibitor of legume ethylene synthesis. Rhizobitoxine reduces host growth, decreasing rhizobia per nodule for all strains on a plant, but substantially increases PHB accumulation for rhizobitoxine-producing rhizobia. In addition to enhancing reproduction, PHB has a role in bet-hedging: when starved, free-living high-PHB rhizobia divide asymmetrically, forming dormant, high-PHB „persisters‟ that survive long-term starvation and antibiotic treatment, and low-PHB „growers‟ that are sensitive to these stresses. Sinorhizobium meliloti integrates bet hedging and phenotypic plasticity, forming fewer high-PHB persister cells when low competitor density predicts shorter-term starvation. Declining populations may select for delayed reproduction when there is a trade-off between reproduction and longevity, as there is with starving S. meliloti.
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    Terminal differentiation of symbiotic rhizobia in certain legume species and its implications for legume-rhizobia coevolution.
    (2010-08) Oono, Ryoko
    The symbiotic association between legume plants (Fabaceae) and nitrogen-fixing rhizobia is a classic system of cooperation, but with largely unexplored differences among species in life history traits. Rhizobia transform physiologically and morphologically into nitrogen-fixing bacteroids inside host nodules. The transformation is terminal (bacteroids are swollen and apparently nonreproductive) in some legume host species but not others, regardless of rhizobial genotype. The phylogenetic distribution of this host trait in the Papilionoideae subfamily of legumes suggests that the common ancestor of the papilionoids did not host terminally differentiated bacteroids and there appear to have been at least five independent origins of hosts imposing terminal differentiation on bacteroids. To consider possible advantages of this host trait, I compared the symbiotic efficiency of terminally and non-terminally differentiated bacteroids of a single rhizobial strain with dual-host capabilities. In the two available dual-host cases, I found greater fixation efficiency (N2 fixation per CO2 respiration) as well as plant return (host biomass) on investment per nodule mass in the hosts with terminal bacteroid differentiation than in those without. This suggests that host traits leading to terminal bacteroid differentiation may have been derived multiple times because of increased net symbiotic benefits to the host. Lastly, I tested whether legumes hosting terminally differentiated bacteroids impose sanctions, i.e. reduce benefits to the undifferentiated reproductive clonemates of less-mutualistic bacteroids in the same nodule. Host sanctions could maintain the evolutionary stability of the symbiosis despite "cheaters" - less-mutualistic rhizobia that potentially benefit from the fixation by other rhizobia sharing the same individual plant host. Legume roots were split so that half of each nodulated root system was exposed to nitrogen-free atmosphere (Ar:O2) to simulate cheating and the other half was in normal air (N2:O2). Rhizobial fitness (rhizobia per nodule) was compared between the two halves. A clear host sanctions effect in peas and alfalfa demonstrated that terminal differentiation of bacteroids does not compromise a legume host's ability to sanction. Differences in rhizobial life history suggest various rhizobial symbiotic traits for cooperation and cheating, perhaps leading to different mechanisms in different legume host species that maintain stability of the mutualism.
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    Vachellia Collinsii: Assessing the Below-Ground Dynamics in an Ant-Plant Relationship (UROP)
    (2024-07-30) Liberman, Katrina
    The mutualistic interaction between the bullhorn acacia (Vachellia collinsii) and its resident ants (Pseudomyrmex) is well-documented for its above-ground dynamics, where the plant provides food and shelter to the ants in exchange for protection and vegetation clearance. This study explores the under-studied below-ground impacts of this mutualism, hypothesizing that the ants’ above-ground clearing activities affect fine root mass beneath V. collinsii canopies. Conducted in June 2024 at Estacion Experimental Horizontes in the tropical dry forest of northwestern Costa Rica, this study compares fine root mass within V. collinsii clearings to that in adjacent control sites without V. collinsii. Fine root samples were collected from six cores per site in ten experimental sites. Statistical analyses revealed no significant difference in fine root mass between V. collinsii and control sites (p = 0.246). However, a significant positive relationship was found between clearing size and the difference in root mass (R² = 0.64, p = 0.013), suggesting that larger clearings are associated with greater differences in fine root mass. These results indicate that while V. collinsii clearings do impact below-ground dynamics, the effects are more pronounced with larger clearing sizes.