Native to Northeast Asia, the Pacific oyster (Crassostrea gigas) has been deliberately introduced into North America (early 1900s), Australia (1940s and 1950s) and France
(1970s) where it is now a very lucrative aquaculture species. Its ability to tolerate a variety of different salinities and temperatures, in conjunction with its high fecundity, has made it an economically important species worldwide. In 2008, world aquaculture produced over 600,000 tonnes of Pacific oysters valued at more than one billion US dollars (FAO, 2010). In Australia, Pacific oysters were released in temperate waters at Oyster Harbour, in
southwest Western Australia, and Pittwater, in Tasmania (English et al., 2000). Only the oysters at the Tasmanian site survived. Spatfall, however, was low and in 1953 this population was moved to the more optimal site of Port Sorrell, in Tasmania, where they flourished and spread to other Tasmanian locations (Stasko, 2000). Today, there are several populations of Pacific oyster in Tasmania and, due to illegal introductions, New South Wales (NSW) (English et al., 2000). These, now naturalized oysters, formed the basis of a number of commercial farms in Tasmania, South Australia and NSW. These farms use seed from two main commercial hatcheries that maintain their own mass selection breeding programs and also use some seed from the national family-based selective breeding program. All programs
21 have occasionally introduced additional wild caught naturalized oysters in an attempt to maintain or increase genetic diversity (Ward et al., 2000).
Over the past decade, there has been a move towards family-based selective breeding programs in aquaculture as opposed to mass selection. In many countries, including Australia, the Pacific oyster industry is entirely hatchery-based, meaning that it is in a position to benefit strongly from genetic improvement (Ward et al., 2000; Langdon et al., 2003; Degremont et al., 2010). Current selective breeding programs have been highly successful, particularly in improving growth rate (Ward et al., 2005). The future application of marker assisted selection (MAS) may provide further enhancement, particularly in relation to summer mortality resistance (Ward et al., 2005; Sauvage et al., 2010). Regardless of breeding technique, the maintenance of genetic diversity is fundamental to future improvement.
Determining the level of genetic variation in cultured, naturalized and native oysters will provide baseline information important to breeding programs, such as revealing whether there has been a significant change since introduction and indicate if poorly managed cultured stocks are at risk of inbreeding. Englishet al. (2000) found no significant genetic difference between Australian cultured (mass selected), Australian naturalized and Japanese native populations of Pacific oyster using allozyme markers, suggesting that there had been little genetic loss since the introduction of the Pacific oyster in the 1940s. The allozyme results of Appleyard and Ward (2006) were in agreement with this conclusion. However, Appleyard and Ward (2006) also used eight microsatellite markers to analyse the cultured (mass selected), naturalized and native populations. They discovered a drop in diversity within the cultured stock, which they attributed to the loss of rare alleles and bottleneck effects.
22 was likely to be minimal. The authors recommended continual monitoring of diversity levels to determine if there is further decline. Kimet al. (2008) found a similar drop in diversity in Korean cultured Pacific oysters compared to wild ones using six microsatellite loci. A study by Li et al. (2006) investigated mass selected Pacific oysters within and among five Chinese farms. The results indicated high genetic diversity within cultured Chinese Pacific oysters and, surprisingly, large differences in allele frequencies between northern and southern farms. This suggests that population structuring can occur within this species.
Within closed breeding populations, some level of inbreeding is unavoidable, but this can be managed in family based programs. Oysters are a highly fecund animal, a trait that can increase the risk of inbreeding if breeders only use a small number of individuals as broodstock (Hedgecock et al., 2004). Additionally, Appleyard and Ward (2006) found that the effective number of broodstock was significantly less than the actual number of
broodstock used, which could also result in greater inbreeding depression. This concurred with work by Boudryet al. (2002) who concluded that unbalanced parental contribution could be explained by both non-genetic and genetic effects. Tolerance to inbreeding is relatively species specific. Evanset al. (2004) found Pacific oysters to be sensitive to
inbreeding depression, reporting an 8.8% decrease in average body weight and 4.3% decrease in survival with a 10% increase in inbreeding. Monitoring of inbreeding is highly important within shellfish culture to ensure that a healthy genepool is maintained.
In the Appleyard and Ward (2006) study, microsatellite markers proved more informative than allozymes at detecting genetic diversity in Pacific oysters. There have been numerous microsatellite markers developed for the Pacific oyster (Hubert and Hedgecock, 2004;
Appleyard and Ward, 2006) many of which have been mapped using linkage analysis (Hubert and Hedgecock, 2004; Hubert et al., 2009; Plough and Hedgecock, 2011). Of these, only 15
23 microsatellite markers in five panels have been multiplexed within this species (Taris et al., 2005; Li et al., 2010). Multiplexing allows more rapid and cost-effective analysis. This study aimed to use at least 10 microsatellite markers, analysed within multiplexing suites, to determine genetic diversity and potential inbreeding of native, naturalized and cultured Pacific oysters. In addition to the native Japanese oysters used in the study by Appleyard and Ward (2006), oysters from Korea and France, as well as Australian naturalized and cultured populations, were analysed.