In partial fulfillment of the requirements for the degree of
Doctor of Philosophy in Quantitative Biosciences
in the School of Biological Sciences
Katie MacGillivray
Defends her thesis:
Evolutionary constraints of cooperation and conflict in microbes
Friday, December 13, 2024
1:00pm Eastern
Location: IBB Suddath Seminar Room (1128)
Zoom: https://gatech.zoom.us/j/95409029999
Co-Advisors:
Dr. Will Ratcliff
School of Biological Sciences
Georgia Institute of Technology
Dr. Brian Hammer
School of Biological Sciences
Georgia Institute of Technology
Committee:
Dr. Sam Brown
School of Biological Sciences
Georgia Institute of Technology
Dr. Frank Rosenzweig
School of Biological Sciences
Georgia Institute of Technology
Dr. Tiffany Lowe-Power
Department of Plant Pathology
University of California Davis
Abstract:
Microbial evolution is heavily influenced by social interactions between and within species. This thesis uses experimental evolution to explore adaptive pathways to defense against attack in bacteria and aggregative multicellularity in yeast.
Bacteria often live in dense multispecies biofilms, and have evolved many antagonistic behaviors they use to kill competitors in the environment. One example is the Type VI Secretion System (T6SS), a molecular harpoon used to stab neighboring cells and inject them with a cocktail of distinct toxins, known as effectors. While the structure and function of the T6SS is well understood, less is known about how T6SS-mediated killing shapes the evolution of targeted bacteria. The T6SS is widespread and deadly, found in ~25% of all Gram-negative genomes with survival rates as low as 10-5. It would therefore be advantageous for bacteria to evolve ways to defend against T6SS attack. This thesis presents our work evolving higher rates of survival in Escherichia coli populations subjected to repeated T6SS attack by Vibrio cholerae. We found that mutations increasing survival often affect membrane and stress response genes. Such mutations come at a cost to growth rate, a major factor preventing widespread resistance to the T6SS. When we extended the period of evolution in the presence of T6SS attack from 500 to 1000 generations, E. coli populations doubled their T6SS survival at no additional cost to growth, likely through one or more additional mutations that compensate for growth costs. By contrast, when evolved populations continued to evolve in the absence of T6SS attack, resistance decreased in favor of faster growth. Evolution of resistance was also dependent on number and type of effectors used by the attacking strain.
The second project of this thesis explores the evolution of multicellularity using experimental evolution of yeast. Multicellular groups that form via aggregation face challenges in evolving group-level traits due to genetic conflict within groups. Here we report on aggregative yeast populations that were evolved for larger group size, and how their increase in assortment allows them to join groups more efficiently than the ancestor. In some cases, this was achieved through mutations in the coding sequence of FLO1, the adhesin they use to attach to one another. We also found one case of kin recognition, where evolved cells preferentially interact with one another rather than the ancestor within the spatial structure of a group in co-culture.
This thesis advances our understanding of the social evolution of microbes, with a focus on factors constraining the evolution of T6SS defense and aggregative multicellularity.