Skelly, Lauren EABSTRACT Title of dissertation: MECHANISMS OF SEXUAL MODE EVOLUTION IN CAENORHABDITIS ELEGANS Lauren Skelly, Doctor of Philosophy, 2022 Dissertation directed by: Professor Eric S. Haag, Department of Biology The evolution of phenotypic novelties is a broad biological phenomenon, and how organisms evolve new traits is dependent on the molecular mechanisms that underlie those traits. Transcriptional regulation is often the focus of phenotype evolution, but post-transcriptional mechanisms such as mRNA splicing, stability, and translational control are also important components. Germ cells are particularly influenced by post-transcriptional mechanisms and are therefore a good system to study how these mechanisms lead to phenotypic novelties. This topic can be studied especially well in model systems that contain closely related species with recently evolved traits, such as self-fertility in Caenorhabditis elegans hermaphrodites. C. elegans hermaphrodites are essentially XX females that evolved the ability to produce sperm in an ovary. There are many known components necessary for sperm development, including a protein-protein-mRNA complex consisting of GLD-1, FOG-2 and tra-2. GLD-1 and FOG-2 dimerize and bind the 3’ UTR of tra-2 mRNA to repress its activity during spermatogenesis. In this work, I show FOG-2 directly interacts with TRA-2 protein, leading to the model that FOG-2 targets TRA-2 protein for degradation during its translation allowing sperm to form. I also show the expression pattern of tra-2 mRNA in the germline, and that GLD-1 binding on the 3’ UTR does not influence its localization. Another method for the study of novel traits is through hybridization of closely related species. In this work, I attempt to hybridize two closely related species of Caenorhabditis, another self-fertile species C. briggsae and an outcrossing species C. nigoni, to map the genetic loci underlying self-fertility. These hybrid crosses are unable to map genetic loci because males are inviable. These results agree with previous studies. This work contributes to the study of phenotypic evolution by adding an underlying molecular mechanism to Caenorhabditis sex determination.enDissertationEvolution & developmentDevelopmental biologyMolecular biologyCaenorhabditisnovel phenotypeself-fertilitySex determinationSexual mode evolution