Genetic And Phenotypic Analysis Of Grandchildless Loci In Caenorhabditis Elegans PDF Download

A common problem in the study of human disease genetics is that the severity of symptoms can vary greatly between individuals, even when the underlying disease mutations are identical. Natural variation in genetic background is recognized as one of the contributors to variation in phenotype severity, but our understanding of the underlying mechanisms is limited. Here, I endeavour to explore why differences in genetic background lead to phenotypic variations by systematically examining a large set of single-gene perturbations in Caenorhabditis elegans. In the first section of this thesis, I broadly examine the effect of genetic background on these perturbed phenotypes employing RNAi in two C. elegans isolates, the canonical lab strain N2 and a commonly used wild-type CB4856 for comparison. This screen yielded 120 genes that produced a more severe phenotype in N2 and 127 genes that produced a more severe phenotype in CB4856 (p-value ≤0.05), with notable enrichments in specific functional groups. These differences in phenotypic severity are recapitulated in both genetic mutants and animals chemically treated to perturb the same functional networks. In the latter part of my thesis, I explore the genetics of variation in perturbed phenotypes and find that they are complex, affected by both the expression level of the gene itself and that of genes within the same functional group. For instance, a gene that has lower expression in N2 will likely exhibit a more severe phenotype in N2, and experimentally lowering the gene expression of the gene itself or genes within the same functional group will increase the severity of the phenotype. I further explore the genetics underlying the phenotypic variation of a subset of these gene perturbations using quantitative trait loci (QTL) mapping. With this approach, I identified a broad trend of four QTLs that repeatedly underlie the phenotypic variations in a set of functionally unrelated genes and that most perturbation variations between N2 and CB4856 have more than one underlying genetic cause. Together, these data show that genetic backgrounds frequently modify perturbed phenotype severities and that the genetics underlying this variation is complex.