Genotypic and Phenotypic Evolution in Experimental Microbial Populations: Causes and consequences of an evolutionary reversal across a major transition

Abstract

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.