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In partial fulfillment of the requirements for the degree of
Doctor of Philosophy in Quantitative Biosciences
in the School of Physics
Seyed Alireza Zamani Dahaj
Defends his thesis:
De novo evolution of macroscopic multicellularity and trait heritability in major transitions.
Tuesday, May 4, 2021
3:00pm Eastern Time
Via BlueJeans: https://bluejeans.com/917049243
Advisor:
Prof. Peter J. Yunker
School of Physics
Georgia Institute of Technology
Co-Advisor:
Prof. William C. Ratcliff
School of Biological Sciences
Georgia Institute of Technology
Committee Members:
Prof. Daniel B. Weissman
Department of Physics
Emory University
Prof. Raphael F. Rosenzweig
School of Biological Sciences
Georgia Institute of Technology
Prof. Joshua S. Weitz
School of Biological Sciences
Georgia Institute of Technology
Abstract:
Evolutionary Transitions in Individuality (ETIs) describe the history of the increasing complexity of life and the emergence of hierarchical organization in an elegant framework. Each transition is characterized by a group of independent individuals coming together and forming a group that eventually can undergo Darwinian evolution and turns into a new individual level. One of the prominent examples of ETIs is the emergence of multicellularity. In this thesis, I address two key questions about the transition to multicellularity: The emergence of heritability of higher-level traits and its relationship to cell-level traits. First, I discuss how the heritability of newly-formed group traits emerges as groups emerge. We introduce a simple theoretical model for calculating group-level trait heritability, where the trait is the linear function of a cell-level trait. For cases in which the relationship is more complex than a linear function, we developed a statistical simulation to model and explore different kinds of analytical functions based on biological examples of relationship between cell-level traits and collective-level traits. Finally, using the snowflake yeast model system we did an experiment that shows an ecologically relevant, emergent trait in a nascent multicellular organism can have a higher heritability across a range of conditions than the unicellular-level trait on which it is based. The evolution of complex multicellularity presents an apparent paradox: nascent multicellular organisms are thought to require (relatively) large size to evolve complex traits, but at the same time maintaining large size requires complex organization at the cell and group levels. This poses a chicken and egg problem between large size and cellular development. Here, we show that over the course of a year snowflake yeast can increase its size multiple orders of magnitude with minimal change at the cell level by taking advantage of the physical properties of granular entangled materials. These results suggest that in the same way that physically imposed geometric constraints can limit the size of clusters, an-other innovations can lead to the emergence of a new biophysical mechanism that changes the material properties of the clusters.
