|Institution:||The University of Montana|
|Department:||Division of Biological Sciences|
|Keywords:||cyanobacteria; nitrogen fixation; thermophile; fitness; adaptation; genomics|
|Full text PDF:||http://etd.lib.umt.edu/theses/available/etd-06302014-145122/|
Understanding population variation for fitness-related traits is important for our comprehension of evolutionary adaptation and of how populations respond to environmental change. Here, I investigate variation in nitrogen fixation performance for an ecologically-variable population of the cyanobacterium Mastigocladus laminosus from White Creek, a nitrogen-limited, geothermally-influenced stream in Yellowstone NP. I next take a population genomics approach to identify candidate loci associated with superior performance. Variation among strains and temperature dependence of the nitrogen fixation process were the most important factors in a linear mixed effects model. Absolute and relative measures of genetic differentiation between strains from the upper quartile of nitrogen fixation performance and the other 75% of strains showed that only a small subset of loci were associated with superior nitrogen fixation. Most notably, the strains that fixed the most nitrogen contained a premature stop codon in a regulatory histidine kinase gene, but this allele was present at low frequency in other strains. Because this nonsense mutation eliminates many important functional sites in the protein, this allele is expected to be non-functional. Both the full-length and the putative null allele, as well as a third recombinant allele, were expressed during nitrogen step-down and in the presence of combined nitrogen. Future studies will investigate whether the nonsense mutation results in transcriptional rewiring that is favorable for nitrogen fixation.