Life cycles and nascent multicellularity
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Marquez Zacarias, Pedro
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Abstract
Life is organized hierarchically. From cells to societies, the levels of biological organization shape evolutionary trajectories. These hierarchies are traditionally studied through the lens of the Major Evolutionary Transitions (MET) framework. One such major transition is the evolution of multicellularity, which gave rise to the evolution of complex life forms like animals, plants, and fungi.
In this thesis, I first discuss how multicellular organisms differ in their ecology and evolutionary trajectories, particularly depending on whether they develop via aggregation (cells coming together) or by clonal development (cells staying together). This will lead up to a more general question about life cycles, and how are these organized differently in multicellular organisms compared to their unicellular counterparts. To give clarity on this matter, I develop a compositional algebra to serve as a formal language to represent and analyze life cycles of arbitrary complexity. With this algebra, I describe the life cycles of complex and simple multicellular organisms, finding that organisms that develop via aggregation are more similar to unicellular organisms than to even simple multicellular organisms. I provide some criteria to distinguish between hierarchical levels of reproduction, which I distinguish from the levels in MET by the features of their life cycles.
Then, I present a simple model of clonal multicellular development that explicitly accounts for spatial constraints. In this model, cells have a specific pattern of cell division that can reproduce simple morphological features observed in nascent multicellular organisms. Using this model, I was able to directly query the effects of aspect ratio and lateral cell growth, or ‘side-budding’. These are two traits known to be important for organismal size in snowflake yeast, which is the experimental system we use in the lab to study multicellular evolution. Further, I show how simple genetic disruptions to the machinery that controls the patterns of growth in single cell yeast can have important effects when applied to snowflake yeast.
Finally, I present a novel method to measure spatial structure in biofilms, which leverages methods and concepts from network theory. This method relies on the network representation of a biofilm, which gives us flexibility and scalability while preserving as much detail as needed. I show how this method is applied to real data, by analyzing microbial communities with cooperative or competitive interactions, and showing the detailed spatial information recovered with the proposed method.
Altogether, in my thesis work I explore the theme of biological organization: the organization of life cycles, the morphological organization of simple multicellularity, and the spatial organization of microbial communities.
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Date
2022-08-24
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Dissertation