Mitochondria are cellular organelles responsible for converting energy from food into a molecular currency useable by the cell. They are usually inherited maternally and possess their own small genome. Over the past decade, there has been considerable debate over the importance of natural selection in influencing the evolution of mitochondrial DNA (mtDNA). Evidence suggests that selection can explain some of the differences in frequencies and kinds of mutations found in comparisons of mtDNA sequences among species. However, this evidence is often confounded by differences in the demographic histories of the species being compared, i.e., stable populations show a different pattern than growing or contracting populations. Our study takes advantage of an unusual pattern of mtDNA inheritance in mussels of family Mytilidae in which two mitochondrial genomes exist simultaneously, one inherited maternally and the other paternally. Since both genomes share a demographic history, differences found between the two mtDNA genomes in our study should provide a much clearer signal of selection. Understanding the role of selection in mtDNA is important in understanding human mitochondrial diseases and other genetically based ailments. In addition, mtDNA is often used as a tool to identify population structure in animals and plants that can be important in identifying appropriate units for conservation.
