Date of Award

Winter 2002

Project Type


Program or Major


Degree Name

Doctor of Philosophy

First Advisor

Thomas D Kocher


Bivalves of the families Unionidae and Mytilidae possess a unique mode of mitochondrial DNA inheritance dubbed sex-limited mitochondrial DNA (mtDNA) inheritance or doubly-uniparental inheritance (DUI). In contrast to the matrilineally restricted inheritance of mtDNA in other metazoans, these bivalves have both matrilineally and patrilineally inherited mtDNA lineages. Although the matrilineally inherited mtDNA (or F type) is present in the somatic tissues of both sexes and the female gonad, the patrilineally inherited mtDNA (or M type) is limited to the male gonad. In Mytilus several observations have been made concerning the evolution of DUI. Firstly, the fidelity of DUI is not perfect; many times in the history of these bivalves the F type mtDNA has invaded the M lineage, resulting in a masculinization event. After invasion the two mtDNA lineages begin divergence de novo. Secondly, the Mytilus M type mtDNA has been shown to evolve more rapidly than the F type and it is hypothesized that this is due to relaxed selection on the M genome. Lastly, the Mytilus F type mtDNA has been shown to evolve more rapidly than other metazoan mtDNA and this is also hypothesized to be due to relaxed selection on the F genome. I examined the evolution of DUI in the freshwater family Unionidae and these results indicate that the mechanisms of DUI are different between the two systems. In the Unionidae, masculinizations are not observed resulting in lineages whose divergence dates to over 450 million years ago. This lack of masculinization correlates with an approximately 600 base pair extension of the cytochrome c oxidase II gene. As with the mytilids the M type mtDNA evolves more rapidly than the F type mtDNA in the unionids; however, this increase in rate of evolution appears to be largely due to male driven evolution and not relaxed selection. Lastly, the increase in rate of the F type Mytilus mtDNA relative to other metazoans is not seen in the unionid lineage; in contrast, the F type mtDNA has a rate of evolution similar to that of other gastropods. This suggests that the rapid rate of evolution of the Mytilus F type mtDNA is not due to relaxed selection, but may be due to other mechanistic forces. The disparity of the unionid system relative to the mytilid system and possible future directions for understanding these differences are discussed.