In partial fulfillment of the requirements for the degree of

Doctor of Philosophy in Biology

In the

School of Biological Sciences

Samiksha Kaul

Will defend her dissertation

Evolution of Polarity Establishment: The Long and Short Story

25/04/2024

9AM

EBB 1005

https://gatech.zoom.us/j/93092869159

 Thesis Advisor:

Dr. Annalise Paaby, Ph.D.

School of Biological Sciences

Georgia Institute of Technology

Committee Members:

Dr. Greg Gibson, Ph.D.

School of Biological Sciences

Georgia Institute of Technology

Dr. Patrick McGrath, Ph.D.

School of Biological Sciences

Georgia Institute of Technology

Dr. Joseph Lachance, Ph.D.

School of Biological Science

Georgia Institute of Technology

Dr. Nic Vega, Ph.D.

Department of Physics

Emory University

ABSTRACT: While early developmental traits are nearly invariant across taxa, the underlying genetic architecture can be complex and significantly diverged.  Because the functioning of the genes involved in such complex processes is often dependent on interactions with other genes, it can be difficult to investigate how the genes, and more importantly, the overall interacting networks evolve. An example of this is the early embryonic process of polarity establishment, whereby dynamically interacting genes work together to establish asymmetric cell division in the early embryo. The genetic and cellular dynamics of this process are well characterized in the C. elegans embryo but how these genes are evolving across different evolutionary timescales is yet to be understood.

This thesis advances what is known regarding the evolution of polarity establishment at the inter- and intraspecific level. By comparing coding sequences as well as variant data, genes involved in polarity establishment were shown to have differed in levels of divergence as they evolved over the Caenorhabditis species phylogeny, and these same genes also exhibited similar patterns of variation within C. elegans, suggesting parallel levels of selection over short and long timescales. A wild isolate of C. elegans was also used to assess how critical polarity establishment genes in a hyperdiverged region vary at the genomic level and the role the genetic background plays in how these genes respond to perturbations. By generating near isogenic lines (NILs) and introducing a genetic perturbation by way of gene knockout in the parents and NILs, it was shown that breaking background interactions can reveal underlying epistasis among genes in the polarity establishment pathway. Furthermore, the importance of using strong quantitative techniques was demonstrated to capture the subtleties of polarity phenotypes in early embryogenesis with the development and implementation of two novel approaches. Natural variation in transcript abundance of critical polarity establishment genes in a hyperdiverged region was revealed by pairing single molecule fluorescence in situ hybridization with a microfluidic chip for high throughput data collection with high spatial and temporal resolution. Also, the effects of a synthetic temperature sensitive mutant of a polarity establishment gene were better characterized by using a temperature control device that provided more precise control of the environment to be able to collect quantitative phenotypic data.