Portland State University. Department of Biology
Date of Award
Master of Science (M.S.) in Biology
1 online resource (vii, 54 pages)
Maintaining mitochondrial genome sequence integrity is essential for preserving normal mitochondrial function. Several human diseases have been associated with heteroplasmic mitochondrial genome mutations, but few genetic systems can simultaneously represent pathogenic mitochondrial genome evolution and inheritance. The nematode Caenorhabditis briggsae is one such model. Natural C. briggsae isolates are globally-distributed and phylogenetically grouped into three distinct clades, with isolates exhibiting varying levels of a large-scale mtDNA deletion, nad[Delta]. Furthermore, a small subset of clade II isolates exhibits putative compensatory mutations that may reduce the risk of deletion formation and accumulation in those populations. In this thesis, I characterize the dynamics of nad[Delta] heteroplasmy levels during both development and transmission in several C. briggsae natural isolates, including two containing putatively protective compensatory mutations (C+).
For all isolates tested, nad[Delta] heteroplasmy levels increased across nematode development, with L1 (first larval stage) exhibiting the lowest deletion load for all but one isolate that exhibited highly variable nad[Delta] levels, while the increase was slowest and overall nad[Delta] levels remained relatively low in C+ isolates. These results support previous work suggesting that nad[Delta] is a selfish element and demonstrate the protective nature of compensatory mutations in inhibiting mtDNA deletion accumulation.
In nad[Delta] inheritance assays, C+ isolates displayed a strong pattern of reversion to wildtype mtDNA levels that was not seen in isolates lacking compensatory mutations (C-). These assays also showed that nad[Delta] inheritance was not well predicted by total maternal nad[Delta] proportion in either C+ or C- isolates; offspring nad[Delta] levels were generally much lower than maternal levels, consistent with some form of negative selection operating between generations. Assays of both maternal somatic and gonadal tissues had slightly more power to predict offspring deletion levels than did assays of whole-worm maternal samples; this result likely points to variance in deletion levels originating from an untested parental tissue type present within the whole-worm samples.
This thesis provided deeper insights into the patterns of mtDNA deletion transmission and age-associated dynamics. It was the first project of its type to survey mutation dynamics and heteroplasmy levels of a naturally-occurring large-scale mtDNA deletion. Thus, my work serves to further develop C. briggsae for use as an experimental model of human mtDNA deletion dynamics and mitochondrial dysfunction.
Sullins, Jennifer Anne, "Accumulation and Transmission Dynamics of a Naturally-Occurring mtDNA Deletion in Caenorhabditis briggsae" (2018). Dissertations and Theses. Paper 4729.
Available for download on Tuesday, December 10, 2019