Date of Award
Doctor of Philosophy (PhD)
The complex social behavior of M. xanthus makes it an excellent model system to study the relationship between genotype and phenotype. Under nutrient rich conditions, a swarm of M. xanthus cells coordinate their movement outward in search of prey. When starved, cells condense into multicellular structures called aggregates. Taken together, these two aspects of the M. xanthus life cycle display several sub-traits that are used to describe its phenotype. Furthermore, the genome of M. xanthus is large, encoding a predicted 7,314 genes, many of which have been linked to aspects of its multicellular phenotype.
This work presented here addresses the genotype-to-phenotype (G2P) problem as it relates to the annotation of a biological process in a model system. The first project addresses G2P from a population genetics approach; we constructed a mutant strain library consisting of 180 single gene knockouts of the ABC transporter superfamily of genes to examine the distribution of mutant phenotypes among an entire group of genes. While the phenotype of only ~10% of mutants show extreme defects, more than three quarters of mutants are parsed into different categories of phenotypic deviation following our analyses. Our results demonstrate that strong mutant phenotypes are uncommon, but the majority of null mutants are phenotypically distinct from wild type in at least one trait. Thus, a more comprehensive understanding of the M. xanthus phenome will help elucidate the biological function of many uncharacterized genes.
The second part of this dissertation examines the evolution of M. xanthus as it has been studied as a model organism in different laboratories. Disrupting a gene, or mutating a single nucleotide, may have no discernable impact on the organism's phenotype by itself, but may still substantially affect the phenotypes of additional mutation through epistasis. This is an ongoing phenomena in M. xanthus; whole genome resequencing of several inter-laboratory isolates of M. xanthus wild type DK1622 reveals genomic variation that has resulted in significant phenotypic variation. We demonstrate that the naturally occurring genetic variants among wild type isolates is sufficient to mask the effect of a targeted mutation in one isolate that is significant in another. These results are the first to indicate that isolates of wild type M. xanthus DK1622 have evolved to a functionally significant degree.
Bradley, Michael, "Evolution, epistasis, and the genotype-to-phenotype problem in Myxococcus xanthus." (2017). Dissertations - ALL. 674.