Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Anderson, Frank


The substantial difference in ionic concentration and osmotic pressure between marine and freshwater environments creates a barrier to dispersal that relatively few metazoan lineages have been able to cross during the evolution of life on earth. Only about half of animal phyla have representatives in both marine and freshwater environments. Even within the phyla that contain freshwater species there are often large clades that continue to be exclusively marine. Interestingly, though, among some of the clades with freshwater species, this transition has occurred repeated. In order to begin to better understand the mechanisms that have allowed some marine lineages to colonize freshwater environments, I investigated the role of gene duplications in this process. First, using published annelid genomes I compared the gene copy number of the Na+/K+-ATPase alpha subunit gene family, the plasma membrane Ca2+ ATPase (PMCA) gene family, and the sarcoplasmic reticulum Ca2+ (SERCA) gene family between marine and freshwater species. I also used gene tree/species tree reconciliation to infer the time of those duplication events. There was a burst of duplications of the Na+/K+-ATPase alpha subunit gene that coincides with the colonization of freshwater habitats by annelids. The evidence of such a burst of duplications for the PMCA or SERCA gene families is inconclusive. Next, in order to increase the sample size and look for more gene families that were involved in the transition to freshwater habitats I downloaded 11 genomes from spiralian animals. I looked for specific gene families that showed a significant increase in size in freshwater species compared to marine species and identified the Na+/K+-ATPase alpha subunit gene family among others. I also used GO enrichment analysis to determine which GO terms were overrepresented in gene families that expanded along freshwater lineages and found terms related to ion transfer to be most common. Finally, I examined available mollusk genomes to compare size of the gene families of interest from the spiralian analyses between marine and freshwater mollusk species. I again found the Na+/K+-ATPase alpha subunit gene family to show a significant increase in size in the freshwater species. How marine animals were able to colonize freshwater habitats is one of the great questions in metazoan evolution and this work represents an important early step in understand this process.




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