Date Awarded

2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Kimberly S. Reece

Abstract

Assessing genetic variation within and among native populations and hatchery stocks of Crassostrea ariakensis using microsatellite markers In response to the dramatic decline of eastern oyster (Crassostrea virginica) populations in the Chesapeake Bay, introduction of the non-native Asian oyster Crassostrea ariakensis has been proposed. Currently several hatchery stocks of C. ariakensis, derived from a few wild populations along the coast of Japan and China, are being maintained in U.S. hatcheries in the Pacific northwest and on the east coast. In recent years, as the risks of the proposed introduction are being assessed, these hatchery animals have been widely used for various research or comparative studies on this species' ecology, biology and disease tolerance, although to date only reproductively sterile triploid individuals have been used for field trials. Genetic concerns have been raised regarding the proposed introduction. There is a need to assess genetic variation within and among native C. ariakensis, determine genetic relationships between existing U.S. hatchery stocks and native populations, and monitor the amount of genetic variation in hatchery populations. In addition, recent studies have revealed that it is very difficult, and indeed often impossible to discriminate among the many species of Crassostrea sympatric with C. ariakensis using morphological features. In fact, a newly described species, Crassostrea hongkongensis, is commonly found to coexist with C. ariakensis in southern China, and, as determined by molecular genetic identification of samples collected for broodstocks and research purposes, these two species are often confused. The genetic research presented here includes a comprehensive study of genetic variability within and among the hatchery stocks in the U.S. and wild populations in Asia using a novel set of microsatellite markers developed specifically for C. ariakensis. In addition, a laboratory hybridization trial was conducted for C. ariakensis and C. hongkongensis, as there are questions regarding the species status of these two taxa. In the hybridization trials, semi gametic incompatibility was observed between C. ariakensis and C. hongkongensis, indicating a partial reproductive isolation between these two taxa. In addition, an order of magnitude higher genetic divergence was observed between these two taxa compared with that found within each taxon based on analyses with three microsatellite markers. The results from the molecular marker analyses, coupled with additional genetic data indicating low transferability of C. ariakensis microsatellite loci to C. hongkongensis and detection of no natural hybrids in samples comprised of thousands of oysters, were consistent with previous sequence phylogeny studies and support the distinct species status of C. ariakensis and C. hongkongensis. . Analysis of C. ariakensis wild populations from the coast of Japan, South Korea, and China based on polymorphisms at eight microsatellite loci found a small but significant genetic differentiation among them, which could be characterized by a genetic pattern of isolation by distance. Eight genetically differentiated populations were further identified across the distribution range of C. ariakensis confirmed to date. Genetic differentiation among five hatchery stocks in the U.S. was five-fold larger than that observed among wild populations in Asia. In addition, significant reduction in genetic diversity compared to wild source populations was observed in these five hatchery stocks, indicating a genetic bottleneck in the stocks. Two mature stocks (TUI and WCA), isolated from their wild source population over 30 yrs, showed greater reduction in allelic diversity (60%) and a significant decrease in heterozygosities (11%--26%) compared to their wild source population, whereas three recent stocks (NCA, SCA99 and SCA00) showed less severe allelic diversity reduction (18%--30%) and nonsignificant change in levels of heterozygosities. These microsatellite markers have proven useful for genetically tracking the origins of C. ariakensis that might be introduced to Chesapeake Bay, particularly for animals originating from hatchery stocks.

DOI

https://dx.doi.org/doi:10.25773/v5-ps7h-6t48

Rights

© The Author

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