In partial fulfillment of the requirements for the degree of

Master of Science in Biology

in the

School of Biological Sciences

Liam Adamson

Will defend his thesis

“Life Cycle Evolution in Long Term Multicellularity Experiment”

April 15th, 2026

1:30 PM (EST) in Cherry Emerson 301A

Thesis Advisor:

Dr. William Ratcliff

School of Biological Sciences

Georgia Institute of Technology

Committee Members:

Dr. Brian Hammer

School of Biological Sciences

Georgia Institute of Technology

 Dr. Peter Yunker

School of Physics

Georgia Institute of Technology

Abstract: 

The transition from unicellular to multicellular life is one of the most consequential innovations in the history of life on Earth. The division of labor and life history trade-offs are essential to this transition, and these aspects shape how a novel multicellular organism allocates resources between growth, structural maintenance, and reproduction. Evolved multicellular yeast from the Multicellularity Long-Term Evolution Experiment (MuLTEE) have been exposed to size-based selective pressure for the last decade, resulting in a range of lineages with differing morphologies, growth rates, and reproductive methods. Early microscopic lineages tend to fracture into evenly sized propagules; however, this physically scaffolded method becomes restricted in subsequent lineages as cells become elongated and entangled — a biophysical innovation that greatly increases structural toughness but renders the fracture-based reproduction mechanism of the early lineages increasingly rare and costly.

To resolve this reproductive bottleneck, I hypothesize that macroscopic MuLTEE lineages have utilized programmed cell death, which severs cell connections from unentangled regions to release small propagules at the cost of cluster mass. Cluster size distributions display a dual peak in the T1000 lineage, indicating that these large, evolved clusters release numerous small propagules. Additionally, cell death measurements reveal increased death rates in evolved lineages, suggesting macroscopic lineages developed an intentional apoptosis-mediated reproductive mechanism. Finally, mixed competitive experiments reveal  propagules survive size-based selection and increase in population-level biomass proportions despite being selected against. 

Together, these findings support the interpretation that apoptosis-mediated propagule production in macroscopic MuLTEE lineages is an adaptive life history innovation. Large, entangled clusters invest heavily in somatic structure to maximize settling performance while generating many small propagules whose collective fitness contribution outweighs their individual disadvantage under selection. This work provides the first direct experimental evidence that this reproductive strategy confers a quantifiable fitness advantage in the MuLTEE and contributes to a broader understanding of how life history trade-offs between offspring quantity and survival shape the early evolution of multicellular complexity.