Browsing by Subject "historical contingency"

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    Fungal Endophytes as Priority Colonizers Initiating Wood Decomposition
    (2016-06-23) Song, Zewei; Kennedy, Peter; Liew, Feng Jin; Schilling, Jonathan; schillin@umn.edu; Schilling, Jonathan
    Priority effects among wood decomposers have been demonstrated by manipulating fungal assembly history via inoculations in dead wood and then tracking community development using DNA sequencing. Individual wood-degrading fungi have been shown, however, to initiate decay after having colonized living trees as endophytes. To track these ‘upstream’ colonizers across the endophyte­saprophyte transition, we coupled high throughput sequencing with wood physiochemical analyses in stem sections extracted from healthy birch trees (Betula papyrifera; 4-7 cm dia.). We incubated wood in microcosms, limiting communities as endophytes­only or challenging endophytes with Fomes fomentarius or Piptoporus betulinus at high exogenous inoculum potential. Initial fungal richness in birch stems averaged 143 OTUs, and decreased nearly three-fold after five months of decomposition. Although F. fomentarius successfully colonized some stem sections incubated at 25°C, decayed wood was generally dominated by saprophytic fungi that were present originally in lower abundances as endophytes. Among saprophytes, fungi in the brown rot functional guild consistently dominated, matching wood residues bearing the chemical hallmarks of brown rot. Despite this functionally redundant outcome, the taxa that rose to dominate in individual sections varied. Surprisingly, the brown rot taxa dominating wood decomposition were better known for lumber degradation rather than log decay in ground contact. Given the isolation from colonizers in our design, this redundancy of brown rot as the outcome suggests that these taxa and more generally brown rot fungi could have adapted to decompose wood where there is lower competitive pressure. Competitive avoidance would complement the diffuse depolymerization mechanisms of brown rot fungi, which are likely more prone to sugar pilfering by other organisms than the processive depolymerization mechanisms of white rot fungi. Overall, this guild-level predictability of fungal endophyte development and consequence is encouraging given the challenges of predicting wood decomposition, and it provides a base for testing these dynamics under increasing natural complexity.
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    Genotypic and Phenotypic Evolution in Experimental Microbial Populations: Causes and consequences of an evolutionary reversal across a major transition
    (2022-05) Khey, Joleen
    Multicellularity is an evolutionary transition which opened up new avenues for adaptation that were inaccessible to unicellular life forms. In this dissertation, I outline one of the few sets of experiments where the effect of history, chance, and adaptation have been studied across a major evolutionary transition – the evolution of multicellularity. I carry out experimental evolution studies to elucidate the extent to which history can feedback and influence future evolutionary trajectories. It has been shown that unicellular yeast can evolve multicellularity by selection for rapidly falling to the bottom of a test tube (“settling selection”). Previously, yeast lines were selected for size, resulting in multicellularity and then selected on agar plates, resulting in reversal to unicellularity. The three experiments described in this dissertation start with these secondarily unicellular yeast strains. Using the same selection scheme described above, I select for reversion to multicellularity. In general, there is quicker reversion to multicellularity in the second round of evolution compared to their naïve unicellular ancestor. There is also strong parallelism in the tempo of reversion among replicate populations, but not between lineages. The genetic basis for the reversion to multicellularity was also evaluated. Differences in genetic basis for the reversion to multicellularity compared to the initial selection experiment demonstrate the importance of historical contingency on the genotypic level. In this first round of settling selection, multicellularity is a single-locus trait, however, after the second round, multicellularity was polygenic. Finally, I examine a surprising consequence of history – the emergence of phenotypic plasticity in a secondarily unicellular isolate. This is one of the few experimental evolution studies on phenotypic plasticity. I show that there is a trade-off associated with the plastic phenotype. Extended experimental evolution to select for further plasticity yielded minimal improvements suggesting that there are evolutionary constraints to the evolution of this phenotype. This research allows us to gain a better understanding of how previous historical events can influence evolution and the predictability of evolution at both the phenotypic and genotypic levels. Historical contingency has far-reaching phenotypic and genotypic consequences, which add to the complexity inherent in biology. This is evident in a system as simple (or as complex) as laboratory yeast subject to falling to the bottom of a test tube.