Quantitative Biosciences Thesis Proposal
School of Biological Sciences
Thesis Advisor: Dr. William Ratcliff (School of Biological Sciences)
Open to the Community
Fitness and phenotype dynamics during the transition to multicellularity.
Friday, September 28th 2018, 1:00 p.m.
Molecular Sciences and Engineering (MoSE), Room 1201A
Dr. Peter Yunker (School of Physics, Georgia Institute of Technology)
Dr. Samuel Brown (School of Biological Sciences, Georgia Institute of Technology)
Dr. Eric Smith (Earth-Life Science Institute, Tokyo Institute of Technology, and
School of Biological Sciences, Georgia Institute of Technology)
The evolution of life on Earth is marked by a few biological innovations that profoundly changed downstream evolutionary trajectories. John Maynard Smith and Eörs Szathmáry termed these innovations Major Evolutionary Transitions and among others, they include the evolution of multicellular organisms from unicellular ancestors. I propose to study this evolutionary transition using experiments and theory. In particular, I will study the mechanisms by which multicellular organisms stabilize their evolutionary trajectories and increase complexity. A first aim will be to study fitness decoupling, which has been proposed as a mechanism to stabilize multicellularity at its onset, limiting the potential for reversion to unicellularity. Little is known about how fitness decoupling arises in evolution, and major questions remain: does it require a trait of large effect, like germ-soma differentiation, or can it arise gradually as a result of multicellular adaptation? I will directly examine this process through experimental evolution in the ‘snowflake yeast’ model system of multicellularity. I expect that multicellular adaptation, arising as a result of strong multicellular-level selection, is both necessary and sufficient for the evolution of fitness-decoupling traits. Another aim is to study the emergence of primitive body plans as a consequence of cell-level division asymmetries, by controlling the axis of cell division in multicellular strains. This will have the purpose to better understand the cellular mechanisms allowing an increase in multicellular size, a key component of multicellular adaptation in our system. Finally, I will study the origins of cell differentiation in a minimal individual-based model that resembles the development of primitive multicellular organisms. For this, I will focus on the role of cell-cell communication in the generation of novel cell behaviors. In this proposal I will present preliminary results for the three aims described above and I will discuss future directions for this research. The understanding of multicellular evolution can inform us about the mechanisms underlying other major evolutionary transitions, and more generally, the outcomes of this proposal can deepen our understanding of the evolution of biological complexity.