xii, 86 p. : ill. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / My dissertation has focused on identifying functions for non-essential genes in essential process, using the early C. elegans embryo as a model system. The fully sequenced C. elegans genome contains ∼19,800 protein coding genes of which about half have identifiable homologs in humans. Classical forward genetic mutagenesis screens, and more recently, genome-wide RNA interference (RNAi) screens has led to the identification of most essential genes in the genome. Analysis of the phenotypic data from mutants and RNAi screens shows that roughly 15% of the genes are essential and an additional 15% produce some other easily identifiable knockdown phenotype. This leaves about 70% of genes that have no functional information. Genetic modifier screening allows for the identification of roles for genes that do not produce a loss of function phenotype on their own but are able to modify the phenotype of a specific mutant. In my first chapter, I introduce approaches to identifying new gene functions and the usefulness V of C. elegans as a model system in this pursuit. In Chapter II, I describe a type of high-throughput genetic modifier screen that combines the sensitized genetic background of temperature-sensitive (ts) embryonic lethal mutants, and RNAi, to identify genes that either enhance or suppress embryonic lethality seen in the mutant background. I also summarize results from screening four ts mutants using this method. The following two chapters describe the identification and characterization of genetic modifier genes for two different ts embryonic-lethal mutants. Chapter III describes modifiers of rfl-1 , a conserved gene required for proper cytoskeletal regulation in the early C. elegans embryo. Chapter IV describes modifiers of lit-1 , also a conserved gene, that is required for regulation of Wnt signaling and cell fate specification in C. elegans . These findings reveal novel genetic interactions and provide functional information about many conserved but non-essential genes that have had no previous characterization. Conclusions are also made about the effectiveness of ts mutant/RNAi screening in the pursuit of identifying new gene functions. This dissertation contains co-authored material that has been previously published, and material that is currently in review, or is being prepared for publication. / Adviser: Bruce Bowerman