Emma Bingham, QBioS Thesis Proposal

Quantitative Biosciences Thesis Proposal 
Emma Bingham 
School of Physics 

Nascent multicellular organisms overcome problems of large size via emergent biophysical phenomena 
Monday, September 25, 2023, at 2:00 pm 
In Person Location: IBB 1128 
Zoom Link: https://gatech.zoom.us/j/91654237489?pwd=K1BwWktITE0vU0I2WDJ2d2FxZmVEdz09 
Open to the Community 

Advisors: 
Dr. Peter Yunker (School of Physics), 
Dr. William Ratcliff (School of Biological Sciences)

Committee Members: 
Dr. Saad Bhamla (School of Chemical and Biomolecular Engineering) 
Dr. Jennifer Curtis (School of Physics) 

Abstract: 
Pressure to become larger is thought to be a driver of the evolution of multicellular organisms. Large size can help an organism avoid predation, resist stress, and use resources more efficiently, among other things. However, though size can solve many problems, it also creates new ones, and it is not clear how nascent multicellular organisms overcome these problems, since they are simple clumps of cells that lack the group-level adaptations of established organisms. I propose to study this issue in two parts.

Emergent physical phenomena can circumvent the need for multicellular adaptations. One problem with large size involves nutrient limitation: nutrients usually cannot penetrate more than 100µm at most into a group of cells, meaning that cells on the inside of a large group will be starved, and growth will be limited. However, our model organism for early multicellularity, snowflake yeast, defies these uptake limits. Over 1,000 days of selection for large size, these yeast evolved to grow exponentially to millimeter sizes, far larger than previously-demonstrated uptake limits. Snowflake yeast does not have cilia to move fluid around, nor does it have complex multicellular adaptations like a circulatory system. Instead, cell-level traits produce emergent physical phenomena that enable it to flow nutrient-rich fluid throughout its body. In Aim 1, I will show that the organism's metabolism drives a rapid, long-range buoyant flow that can feed all of its cells. In Aim 2, I propose to show that its porous geometry, created by elongated cells that entangle with one another, is in a physical `sweet spot' for the range of flow speeds it experiences both in this metabolism-driven flow and in a shaken tube. Together, these physical phenomena help snowflake yeast overcome nutrient limitation without the need to develop any complex multicellular adaptations.

When does size lead to morphological complexity? Not only is size a driver of multicellularity, but it is also thought to be a precursor to complexity. Many large, established multicellular organisms are highly patterned, with multiple cell types. However, if as proposed above, larger size can be achieved without morphological complexity, then under what conditions would a nascent multicellular organism become more complex? In Aim 3 of my proposal, I will ask whether emergent biophysical adaptations promote or inhibit the evolution of more morphologically complex groups of cells with patterning or division of labor. I will develop models inspired by snowflake yeast and other organisms such as Volvox to understand how different structural solutions to problems of large size might be selected. This proposed work will help contextualize the role of physics in the evolution of multicellularity.