Date Awarded


Document Type


Degree Name

Master of Science (M.Sc.)


Virginia Institute of Marine Science


Ryan B Carnegie

Committee Member

Andrew Wargo

Committee Member

Jeffrey Shields

Committee Member

William Walton

Committee Member

Erik Sotka


Parasites and hosts are in a constant arms race with one another. Due to their proximity, changes in a host gene can stimulate an evolutionary change in the parasite's genes, which can prompt another evolutionary change in the host, and so the cycle continues as per the well known Red Queen Hypothesis. This interaction is seen in many organisms everywhere and has significant implications for the survival of many keystone species, ecosystem engineers, and organisms that are an important food source to both humans and animals. The host-parasite interaction between the eastern oyster, Crassostrea virginica, and the parasite Perkinsus marinus likely represents one such example. The eastern oyster is an ecosystem engineer, providing a habitat that many invertebrate and fish species depend on, and is a top cultured species in the United States. However, when P. marinus intensifies in its host species in Chesapeake Bay, C. virginica populations can diminish, jeopardizing the survival of organisms that rely on them and their support of an important industry. In response, C. virginica may evolve greater tolerance for P. marinus which allow the oyster to persist despite P. marinus prevalence. It is also possible P. marinus will itself rapidly change phenotypically and genotypically and continue to cause a threat to eastern oyster populations. This thesis is a two-part study looking at the current state of the ecology and evolution of this interaction. First, a field study and survey analysis from Chesapeake Bay, Virginica revealed that P. marinus was the primary contributor to oyster mortality, causing about 40% mortality of wild adult oysters from one population in the York River in 2021, below historical highs of over 70%. Both oyster mortality and Haplosporidium nelsoni prevalence have steadily decreased since 1989, although P. marinus prevalence and infection intensities have remained relatively high. The reduced mortalities despite persistent P. marinus infections provides further evidence for tolerance adaptation to P. marinus in the eastern oyster. Second, I used genotype-by-sequencing as a tool to analyze the population genetic structure of P. marinus and C. virginica simultaneously for the first time. Our approach generated 52,458 SNPs (single-nucleotide polymorphisms) from C. virginica and 105 SNPs from P. marinus from 32 individuals from four populations in the Gulf of Mexico and Atlantic coasts. There was a strong phylogeographic break between Gulf of Mexico and Mid-Atlantic coast populations, and between the outer coast and Chesapeake Bay of Virginia for both species, suggesting that the host and parasite have similar phylogeographic structure within and between regions. Additional analyses indicated there are rare long-distance migrations of parasite genotypes between the Gulf and Atlantic, likely due to anthropogenic movement of oysters. Collectively, these studies provide insight into how the interaction between C. virginica and P. marinus has changed and suggest hypotheses for the future of this host-parasite interaction.



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