Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Stan Allen

Committee Member

Jan McDowell

Committee Member

Louis Plough

Committee Member

Emily Rivest

Committee Member

Jessica Moss Small


Triploids are a popular product in commercial oyster aquaculture and make up most of the hatchery-produced Crassostrea virginica farmed in the Chesapeake Bay. Despite their importance to commercial aquaculture, the potential of genetically improving triploid C. virginica from selective breeding and breeding strategies for their improvement had not been evaluated. In this dissertation, the prospect of improving triploid C. virginica through selective breeding was assessed with a quantitative genetic analysis from a field test, and breeding strategies for genetically improving triploids were compared by computer simulation. Heritability and genetic correlations involving commercial traits in triploids, including mass mortality associated with late spring conditions, or “triploid mortality,” were estimated from twenty paternal half-sib triploid families and forty full-sib tetraploid families reared at three sites in the Chesapeake Bay. A triploid mortality event only occurred at a site on the bayside of the Eastern Shore of Virginia (Nandua Creek), with three triploid families having survival less than 0.70 between May 7 and July 9. The heritability of survival in triploid families during the triploid mortality event was high (1.06 ± 0.32), suggesting selective breeding can reduce the risk of these mortalities in the future. Genetic correlations between survival in triploids at Nandua Creek and the other two sites, York River and Choptank River, were low (0.46 ± 0.22, 0.46 ± 0.24), indicating a weak relationship between genes causing “triploid mortality” and genes causing mortality at York River and Choptank River. Heritability for total weight, meat weight, and shape traits in triploids was often high (> 0.30) and higher than that previously reported for diploid C. virginica. Genetic correlations between traits in triploids and tetraploids were always positive and ranged from 0.30 to 1. Although the positive genetic correlations indicate that selecting for genetic improvement of tetraploids will also lead to genetic improvement in triploids, the estimates had high standard errors, leaving the strength of the relationship unclear. Breeding strategies for genetically improving triploids were compared by simulation with a focus on the effect of genetic correlations between ploidies. The strategies were 1) separate diploid and tetraploid family breeding programs, without phenotyping triploids and 2) a single family breeding program phenotyping diploids, triploids, and tetraploids. The strategy of phenotyping all ploidies resulted in more genetic improvement of triploids when between-ploidy genetic correlations were low (0.33 – 0.66), and the two strategies had similar results at higher genetic correlations (0.75 – 0.90). The higher or similar improvement of triploids across moderate genetic correlations suggests the single breeding program is the better approach, especially if robust estimates of between-ploidy genetic correlations are unavailable. Potential exists to genetically improve triploid C. virginica in the Chesapeake Bay through selective breeding, including reducing the risk of “triploid mortality.” Phenotyping diploids, tetraploids, and triploids in a single breeding program is likely to yield the highest possible improvement in triploids if using family selection. Future studies should assess the benefit of applying genomic selection to polyploid oyster breeding. Genomic selection may be highly advantageous for improving triploids because it could enable identification of individual diploids and tetraploids that have the highest genetic value for triploid production.




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