Date Awarded


Document Type


Degree Name

Master of Science (M.Sc.)


Virginia Institute of Marine Science


The effect of triploidy to effect faster growth and increased survival compared to diploid oysters is called the ‘triploid advantage’ and this advantage in the Chesapeake Bay for Crassostrea virginica is the principal reason for the value of triploid C. virginica in the Chesapeake Bay, USA. The triploid advantage is hypothesized to be the result of genetic effects, physiological changes, or a combination of both. The causative genetic mechanisms at play may include additive genetic effects and heterosis while the physiological changes obtain from triploid sterility. The triploid advantage was examined by comparing 13 diploid and 13 triploid crosses across three environments. The genotypes used in this study consisted of wild stocks from both the Virginia and Maryland portions of the Chesapeake Bay, four lines from ABC’s 2006-year class of selected lines, and four ABC Superlines. Three experimental sites, ranging in salinity and disease pressure (Choptank River – low salinity and no disease pressure; Rappahannock River – moderate salinity and occasional disease pressure; York River – higher salinity and consistent disease pressure) were chosen to investigate the influence of environment on triploid advantage. Growth metrics (shell height, whole wet weight, and wet tissue weight) and survival rates among diploid and triploid C. virginica were recorded. The triploid advantage for growth and survival ranges from positive to negative depending on environmental factors. In the low salinity environment, triploidy proved disadvantageous regardless of the genotype of the diploid parent. As salinity increased so did the triploid advantage, which was greater for the more disadvantaged (wild) groups. In the Rappahannock River, with moderate salinity and no disease pressure, selected diploids performed equivalently to their triploid counterparts showing that breeding efforts can improve diploid field performance to rival triploids. In the York River, under disease pressure, triploids offered the greatest advantage. Triploids from both wild-type and selected diploids had higher growth and survival than their diploid counterparts under disease pressure. In addition to greater survival, triploids also had lower Dermo infection prevalence than diploids indicating that there is a triploid advantage for Dermo disease resistance, perhaps as a result of triploid sterility. Variation in the effect of triploidy on field performance follows the notion that triploidy may be thought of as a tool useful in some applications but not in others. For low salinity, it appears that triploidy may not be the appropriate tool for providing benefits for oysters but for oysters grown under disease pressure, it certainly is. The triploid advantage appears to be caused by both genetic effects and physiological changes, with the environment influencing the expression of each in manifesting the triploid advantage.



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