Characterizing the Role of DNA Methylation Patterns in the California Mussel, Mytilus californianus

The role of DNA methylation is widely understood in vertebrates. However, recent studies reveal an opposing pattern and function for DNA methylation in some invertebrate taxa. In invertebrates, methylation has been observed in coding regions of DNA and is postulated to protect conserved genes, through hypermethylation, from variations induced by the environment. Conversely, limited methylation of inducible genes affords them environmental plasticity by allowing for the creation of alternative transcripts. Absent from prior invertebrate studies are marine intertidal organisms that experience stressful environments. Similarly, DNA methylation may offer protection to critical genes while affording physiological plasticity to inducible genes in intertidal organisms. Therefore, I investigated the pattern and potential roles of DNA methylation as well as its potential to contribute to locally adapted phenotypes in the California mussel, Mytilus californianus. The results from a reciprocal transplant experiment of two mussel populations that experience ecologically different environments suggest these populations can exhibit locally adapted phenotypes. Given that these populations are believed to be genetically homogenous, the differences observed in global methylation may permit locally adapted phenotypes while allowing the underlying DNA to remain genetically homogenous. Bisulfite sequences of genes found in differentially expressed categories of these populations reveal methylation may be altered as a function of environment. From in silico analyses and bisulfite sequencing, I determined hypermethylated genes were enriched in housekeeping functions while hypomethylated genes were enriched in genes with inducible and immune responses. Together, DNA methylation may serve to regulate and protect genes in mussel populations as well as generate phenotypic differences in unique environments.