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The eastern oyster, Crassostrea virginica, is a commercially and ecologically important inhabitant of estuaries along the east coast of North America. Declining numbers of C. virginica due to overfishing, loss of habitat, and disease have led to increased efforts to explore disease resistance and genetic diversity in this species, an essential step in understanding disease resistance and restoring oyster populations. The recent increase in genomic research in C. virginica has provided resources for investigating genetic differences among natural populations.
Typical of many intertidal marine invertebrates, C. virginica has a life cycle with a long larval dispersal period and a sedentary adult stage. Because larval dispersal of C. virginica allows for potential gene flow, the extent of subdivision of this species into genetically distinct subpopulations remains unclear. Previous studies on the geographic variation of C. virginica populations along the Atlantic and Gulf coasts of North America have employed allozymes, mitochondrial DNA, and a limited number of anonymous and nonanonymous nuclear loci to examine population structure. However, markers utilized in these studies have produced discordant results in oyster population genetic data.
Single nucleotide polymorphisms (SNPs) constitute a class of genetic markers that are highly abundant and widespread in genomic sequences of many species. Because the number of loci is virtually unlimited and they are amenable to robust high-throughput genotyping methods that are easily standardized among laboratories, SNPs are rapidly becoming the standard tools for the analysis of population structure, genetic mapping, and taxonomic identification. In an effort to further investigate geographic variation among C. virginica populations, we examined SNPs identified in nuclear loci for use as genetic markers.
The overall goal of this research study was to assess genetic variation among natural populations of C. virginica and utilize this information in the development of genetic markers to elucidate population structure throughout the natural range of this species. This dissertation is divided into four research components. In the first part, I developed PCR primers to amplify 60 oyster nuclear loci in order to identify loci exhibiting variation among natural populations. Sequencing 36 nuclear loci in a set of oysters collected from different geographical locations, I observed a high level of DNA polymorphism among natural oyster populations. Analysis of genetic variation among geographically separated populations of C. virginica revealed regional separation of North Atlantic, South Atlantic, and Gulf of Mexico individuals. In the second part, I mined the sequence data to identify SNPs to use as genetic markers in a more extensive investigation of geographic variation among C. virginica populations. I developed a set of 12 SNP markers in 10 C. virginica nuclear loci, and implemented three cost-efficient, moderate-throughput methods of SNP genotyping. In the third part, I utilized the set of 12 SNP markers to conduct a comprehensive analysis of geographic variation in SNP frequencies throughout the species range to identify genetically distinct subpopulations of C. virginica. Analysis of the 12 SNPs supported the sequence analysis findings of significant genetic differentiation between Atlantic and Gulf coast oyster populations as well as moderate genetic differentiation between North and South Atlantic populations. The last component was an intensive genetic analysis of Gulf coast oyster populations utilizing both mitochondrial and nuclear markers in an effort to better understand the differences in what the different types of markers can tell us about population differentiation in this region.